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ATREATISE 

ON 

THE  FORCES  WHICH  PRODUCE 

THE 

ORGAIIflZATIOIIf  OF  PlAIIfTS. 

WITH  AN  APPENDIX, 

CONTAINING  SEVERAL  MEMOIRS  ON  CAPILLARY  ATTRACTION,  ELECTRICITY,  AND 

THE  CHEMICAL  ACTION  OF  LIGHT. 

BY 

JOHN  WILLIAM  DRAPER,  M.D., 

PROFESSOR  OF  CHEMISTRY  IN  THE  nNIVERSITY  OP  NEW-YORK. 


NEW-YORK  : 
PUBLISHED  BY  HARPER  &  BROTHERS, 

No.   82    C  L  I  F  F- S  T  R  E  E  T. 


1844. 


Entered,  according  to  Act  of  Congress,  in  the  year  1844,  by 

Harper  &  Brothers, 
In  the  Clerk's  Office  of  the  Southern  District  of  New- York. 


PREFACE. 


In  the  autumn  of  1833,  during  a  period  of  convalescence  from  a  severe  attack  of  the  fever  of 
the  Southern  States,  my  thoughts  were  first  turned  to  the  connexion  of  Chemistry  with  Vegetable 
Physiology. 

At  that  time,  as  the  works  on  Chemistry  show,  comparatively  but  little  advance  had  been  made 
in  this  interesting  department  of  science.  In  the  most  popular  treatises,  Organic  Chemistry  consisted 
chiefly  of  a  descriptive  history  of  the  acids  and  bases  of  the  vegetable  and  animal  kingdoms. 

It  is  impossible  for  any  one  to  witness  in  those  warm  climates  the  rapidity  with  which  all  the 
operations  of  vegetable  life  are  carried  on,  without  having  his  thoughts  directed  to  the  obvious  con- 
nexion which  exists  between  these  phenomena  and  external  agents.  Even  in  the  opinion  of  the 
uneducated  planter,  the  rapidity  of  vegetable  growth  is  connected  with  the  warmth  of  the  season, 
and  the  occurrence  of  favourable  rains,  and  the  brilUancy  of  the  sun. 

A  seed  which  has  been  buried  in  the  ground  a  few  days,  makes  its  appearance  above  the  surface. 
It  soon  puts  forth  its  leaves,  which  turn  green  in  the  light,  and  becomes  an  active  laboratory  of  all 
kinds  of  chemical  and  mechanical  processes.  Starch,  gum,  sugar,  and  a  variety  of  other  substances 
are  formed  from  inorganic  matter,  water  is  drawn  up  in  large  quantities  from  the  ground,  and 
evaporated  from  the  leaves. 

All  these  phenomena  unquestionably  depend  on  the  common  laws  of  physics.  Our  interest  in 
them  is  greatly  increased  by  the  close  resemblance  of  many  of  them  to  things  taking  place  in  the 
case  of  animal  systems.  The  productions  of  vegetable  life  are,  many  of  them,  also  apparently  the 
productions  of  animal  life.  The  physical  processes  which  appear  in  one  of  the  great  classes  of 
living  beings,  appear  in  the  other  too.  Inquiries  into  the  nature  of  the  vital  processes  of  plants  end 
in  resolving  problems  connected  with  the  physiology  of  animals. 

In  every  plant  there  are  two  prominent  operations  carried  forward :  the  production  of  organic 
matter,  and  its  distribution  through  the  various  parts  of  the  vegetable  system.  It  is  to  the  consid- 
eration of  these  that  the  following  pages  are  chiefly  devoted.  As  to  the  various  transformations 
which  take  place  in  the  interior  of  these  organisms,  and  the  chemical  principles  involved  in  those 
changes,  anything  which  I  might  offer  would  be  insignificant,  compared  with  the  splendid  results 
which  have  been  obtained  by  the  German,  French,  and  English  chemists  during  the  period  which 
has  elapsed  since  these  researches  were  first  undertaken. 

Any  one  who  peruses  the  works  on  Botany  and  Vegetable  Physiology  must  be  struck  with  the 
inexact  views  which  are  entertained  on  the  mode  of  action  of  the  solar  rays  in  producing  the  green 
colour  of  plants,  and  effecting  the  decomposition  of  carbonic  acid  gas.  Yet  this  is  unquestionably 
the  most  remarkable  result  in  physiological  and  physical  science.  It  is  the  transmutation  of  inor- 
ganic into  organized  matter.  With  a  view  of  giving  clearness  and  precision  to  those  vague  notions, 
I  have  devoted  much  of  the  following  work  to  this  point,  showing  how,  as  plants  grow  in  water  or 
in  air,  they  are  furnished,  upon  phvsical  principles,  with  nutrient  material ;  how,  upon  this,  the  sun- 


PREFACE. 

beam  acts,  and  how,  of  the  rays  of  which  that  beam  is  composed,  the  yellow  ray  of  Hght  is  the 
operative  principle.  The  mode  by  which  the  resulting  decomposition  is  effected  leads  to  an  in- 
vestigation of  the  absorption  of  light. 

As  will  be  seen  from  the  Appendix,  in  these  investigations  I  have  not  restricted  myself  exclusively 
to  the  subjects  before  me,  but  have,  in  most  instances,  followed  them  out  into  their  physical  details. 
The  chapters  of  the  Appendix  have  all  been  published  in  various  American  and  foreign  works,  each 
chapter  being  a  memoir,  more  or  less  complete,  on  the  topic  on  which  it  treats.  In  that  Appendix, 
therefore,  there  will  be  found  a  large  amount  of  experimental  matter,  of  which  an  immediate  use  is 
not  made.  But  in  philosophy  no  new  facts  are  superfluous.  At  first  sight,  there  might  seem  to  be 
no  connexion  between  the  fixed  Unes  of  the  solar  spectrum  and  the  production  of  chlorophyl,  yet,  as 
will  be  hereafter  understood,  very  great  use  may  be  made  of  the  former  in  investigations  on  the 
nature  of  the  latter. 

The  remark  which  has  just  been  made  respecting  the  vague  ideas  which  are  entertained  in  rela- 
tion to  the  mode  of  action  of  light,  might  also  have  been  made  in  relation  to  capillary  attraction. 
This  force,  with  exosmose  and  endosmose,  are  very  favourite  resources  of  the  botanist  in  his  diffi- 
culties. In  the  chapters  in  which  these  topics  are  considered,  I  have  endeavoured  to  show  the 
true  relation  which  exists  between  endosmosis  and  capillary  attraction,  and  how  the  latter  itself 
springs  from  electric  excitement.  I  have  shown  how,  in  strict  conformity  with  this  principle,  the 
flow  of  the  sap  and  the  circulation  of  the  blood  may  be  explained  ;  the  systemic  circulation,  for  ex- 
ample, arising  as  a  necessary  consequence  of  the  deoxydation  of  arterial  blood. 

The  theory  of  these  circulatory  movements  taking  place  in  organized  structures  I  have  not  here- 
tofore published,  though  they  have  been  taught  in  my  lectures  in  this  University.  It  will  be  seen 
that  the  peculiarity  is  in  tracing  the  action  to  chemical  changes  ;  that  the  systemic  circulation  arises, 
as  has  just  been  said,  from  the  deoxydation  of  arterial  blood ;  the  pulmonary  circulation  from  the 
oxydation  of  venous  blood ;  and  the  flow  of  sap  in  flowering  plants  from  the  carbonization  of  water 
by  the  sunlight  in  their  leaves. 

It  might  be  supposed  that  Chapter  XVIII.,  Ap.,  is  intended  as  a  reply  to  the  criticisms  on  some 
parts  of  Chapter  XIII.,  which  have  been  published  by  M.  Edmond  BEcauEREL,  in  the  Annales  de 
Chimie.  When  that  chapter  was  written,  I  had  not,  however,  had  the  opportunity  of  seeing  his 
valuable  memoir.  The  experiments  there  given  were  not  made  with  the  iodide,  but  the  bromide 
of  silver,  a  circumstance  which  relieves  me  from  much  of  the  weight  of  the  criticisms  of  that  chem- 
ist. It  is  not  only  from  my  own  experiments,  but  also  from  those  of  Sir  J.  Herschel,  that  I  am  led 
to  differ  from  M.  Becouerel,  in  regarding  the  iodide  and  bromide  of  silver  as  exhibiting  very  dif- 
ferent changes  in  the  spectrum.  M.  BEcauEREL's  idea  that  there  is  but  little  difference  between  them, 
does  not  seem  to  be  consonant  to  experiment. 

Of  the  Appendix,  several  chapters  have  already  been  translated  into  different  European  lan- 
guages, and  a  variety  of  criticisms  upon  them  have  been  published.  It  is  this  circumstance  which 
has  caused  me  to  hasten  the  printing  of  the  whole  of  these  papers  in  this  collected  form  now  offered 
to  the  public.  Whatever  care  be  taken  in  the  preparation  of  such  translations,  it  will  necessarily 
happen,  that  on  many  points  mistakes  will  be  made  as  to  the  meaning  and  views  of  an  author. 
This  has  happened  in  the  present  instance,  both  in  Germany  and  France :  for  example,  in  Chapter 
XV.,  which  has  been  republished  in  Paris  in  the  Annales  de  Chimie,  errors  of  the  kind  occur. 
Those  who  arc  familiar  with  these  matters  will  find,  when  they  look  over  these  papers  in  the  form 


I 

PREFACE.  y 

now  given,  that  views  have  been  thus  accidentally  imputed  to  the  author  which  he  does  not  enter- 
tain. 

It  did  not  enter  into  my  plan  to  reprint  any  of  the  valuable  memoirs  which  have  been  published 
by  other  chemists  on  these  topics,  more  especially  on  those  relating  to  the  chemical  agencies  of 
light.  I  may,  however,  direct  the  attention  of  the  readers  to  a  description  by  Sir  J.  Herschel,  of 
the  spectrum  alluded  to  in  the  Appendix  (645).  It  is  inserted  in  the  Philosophical  Magazine  for 
February,  1843,  In  this  paper  will  be  found  what  probably  will  prove  to  be  the  true  theory  of  the 
action  of  light  on  Daguerreotype  plates,  and  the  reader  of  this  volume  will  be  the  more  interested 
in  it,  inasmuch  as  it  contains  criticisms  on  several  of  the  leading  doctrines  here  set  forth. 

The  different  chapters  of  the  Appendix  are,  for  the  most  part,  reprinted  from  sources  mentioned 
under  the  title  of  each.  In  a  few  instances  slight  changes  have  been  made  in  them  ;  cha»ges  which 
have  been  rendered  necessary  by  the  mode  in  which  they  were  originally  published.  Thus  some 
of  these  papers  were  printed  in  various  American,  and  others  in  European  journals,  and  it  some- 
times happens  that  experiments  are  in  consequence  described  twice  over.  This  repetition  I  have 
avoided  by  removing  the  paragraphs  involved.  In  this  manner  I  have  entirely  omitted  to  republish 
in  this  work  a  paper  on  the  constitution  of  the  atmosphere,  given  in  the  Philosophical  Magazine  for 
October,  1838,  because  nearly  all  its  experimental  matter  had  been  previously  published  in  the 
American  Journal  of  Medical  Sciences,  and  is  given  in  Chapter  VI. 

The  time  has  now  arrived  when  both  Vegetable  and  Animal  Physiology  are  to  have  their  founda- 
tions laid  on  Chemistry  and  Natural  Philosophy,  the  only  basis  which  can  elevate  them  from  their 
present  deplorable  position  to  that  of  true  sciences.  It  is  with  this  impression  that  the  explanations 
which  I  have  given  in  this  book  of  the  mode  by  which  light  acts  in  determining  organization,  and 
of  the  mechanical  causes  by  which  such  organized  matter  is  transmitted  from  point  to  point  of 
living  systems  (for  these  are  the  leading  facts  which  this  work  is  designed  to  illustrate),  are  offered 
to  the  attention  of  chemical  philosophers. 

John  William  Drapek. 

University  of  New- York,  October  1,  1844. 

N.B. — The  references  to  the  Appendix  are  made  by  the  number  of  the  paragraph,  thus  (Ap.,  100, 
101).  When  the  place  referred  to  is  in  the  body  of  the  work,  the  letters  Ap.  are  omitted,  thus 
(100,  101). 


CONTENTS. 


INTRODUCTION. 

GENERAL  REMARKS   ON  THE  INFLUENCE   OF   PHYSICAL  AGENTS  ON  ORGANIZATION   AND  LIFE. 

Object  of  the  Work  stated. — Connexion  of  Organization  with  the  Imponderable  Principles. — General  Laws 
direct  all  Astronomical  Occurrences  and  Physical  Events. — Transitory  Nature  of  all  Combinations,  and 
especially  those  which  are  Organized. — Time  is  an  Element  of  Life. — Inorganic  Changes  are  brought  about 
by  Physical  Laws,  as  also  is  the  Extinction  of  Living  Races. — Influence  of  Climate  on  the  Distribution 
and  Character  of  Animals  and  Plants. — Relations  between  Animated  Forms  and  the  Atmosphere. — Influ- 
ence of  Currents  in  the  Air  and  in  the  Sea. — Gradual  Emancipation  of  the  Higher  Races  from  the  Direct 
Action  of  External  Agents    .............    Page  1 

CHAPTER  I. 

ON  THE   ACTION  OF  THE   SUNBEAMS    IN  PRODUCING  ORGANIZED  BODIES. 

The  Growth  of  Confervae  in  Water. — Production  of  Green  Matter  by  Spun  Glass  and  Inorganic  Bodies. — 
It  requires  a  Sporule,  Cell,  or  Objective  Germ. — Mode  of  Diffusion  of  Gases  into  Water. — Influence  of 
Temperature  on  the  Process. — Action  of  the  Sun's  Rays  on  these  Gases. — Two  Atmospheres  around  the 
Earth. — Sources  of  Supply  of  the  expended  Gases. 

Application  of  these  Principles  to  the  Production  of  Chlorophyl  in  Leaves. — The  Digestion  of  Plants     .  15 

CHAPTER  II. 

ON  THE   MECHANICAL   CAUSE  OF  THE  FLOW  OF   THE   SAP   IN   PLANTS.      IT  IS   DUE   TO   THE  CARBONIZATION 
OF  WATER   IN   THE   LEAVES   BY  THE  LIGHT  OF   THE  SUN. 

The  Flow  of  Sap  and  Circulation  of  Blood  are  probably  due  to  the  same  Physical  Cause. — Amount  of  Water 
circulating  in  Plants. — Botanical  Theory  of  the  Flow  of  Sap — fails  for  the  descending  Sap. 

Capillary  Attraction  described. — Elevation  or  Depression  of  Liquids  depends  on  their  welting  or  not  wetting 
the  Tube. — No  Flow  in  an  ordinary  Capillary  Tube. — Conditions  for  producing  a  Flow — such  as  Evapo- 
ration, Decomposition,  and  Solution. — Endosmosis  produced  on  these  Principles  by  Solution. — Dutrochet's 
Experiments. — Explanation  of  them. — General  Law  of  these  Movements. — Force  with  which  they  take 
place. — Capillary  Attraction  due  to  Electricity. 

Application  of  these  Principles  to  the  Ascent  of  the  Sap. — Exhausting  Action  of  the  Leaves. — Cause  of  the 
Descent  of  the  Sap. 

The  Light  of  the  Sun  is  the  Cause  of  the  Flow  of  the  Sap  both  in  its  Ascent  and  Descent    .       .  .22 

CHAPTER  III. 

ON  THE   MECHANICAL   CAUSE   OF  THE   CIRCULATION   OF   THE  BLOOD. 

Ancient  Theory. — Description  of  the  Systemic,  Pulmonary,  and  Portal  Circulation. — General  Law  of  Move- 
ment.— Capillary  Relations  of  Arterial  and  Venous  Blood  to  the  Tissues. — The  Systemic  Circulation  is 
due  to  the  Deoxydation  of  Arterial  Blood,  and  its  Direction  is  therefore  from  the  Artery  to  the  Vein. 

Pulmonary  Circulation. — Capillary  Relations  of  Arterial  and  Venous  Blood  to  Atmospheric  Oxygen. — Pul- 
monary Circulation  is  due  to  the  Oxydation  of  Venous  Blood,  and  its  Direction  is  therefore  from  the  Venous 
to  the  Arterial  Side. — Uses  and  Action  of  the  Heart. 

Portal  Circulation. — Capillary  Relations  of  Arterial,  Portal,  and  Venous  Blood  to  the  Liver. — Three  Sources 
of  Force  in  conducting  the  Portal  Circulation. 

Action  in  Asphyxia. — Case  of  obstructed  Trachea  33 

CHAPTER  IV. 

ON  THE  PHYSICAL  CONSTITUTION  OF  THE  SUNBEAMS   AND   ON   THE   PRISMATIC  SPECTRUM. 

Modes  of  isolating  the  Coloured  Rays. — Newton's  Prismatic  Spectrum. — Theory  of  the  Colours  of  Light. — 
Illuminating  Calorific  and  Chemical  Powers  of  the  Spectrum. — Newton's  Processes  for  purifying  the 
Spectrum. — Fixed  Lines. — Melloni's  Experiments  on  the  Distribution  of  Heat. — Physical  Independence 
of  Heat. — Herschel's  Experiments  on  the  Thermic  Spectrum. — Chemical  Action  of  the  different  Regions 
of  the  Spectrum  on  a  Daguerreotype  Plate. — Chemical  Action  on  other  Bodies        .       .       .  .41 

CHAPTER  V. 

ON  THE   INTERFERENCE  SPECTRUM. 

Defects  of  the  Prismatic  Spectrum. — Mode  of  forming  the  Interference  Spectrum. — Its  Peculiarities. — The 
Distribution  of  the  Colours,  and  Law  of  their  Intensities. — Reflected  Interference  Spectrum. — Its  Fixed 
Lines. — Wave-lengths  of  the  Seven  Great  Rays. 


yiii  CONTENTS. 

Melloni's  Researches  on  the  Distribution  of  Heat  in  Perfect  Prismatic  Spectra. — Apparent  Identity  ol  Light 
and  Heat. 

Distribution  of  Chemical  Force  in  the  Interference  Spectrum. — Comparison  of  the  Fixed  Lines  in  the  Pris- 
matic and  Interference  Spectrum. — Mode  of  Defining  Chemical  Effects  by  Wave-lengths  or  by  Times  of 
Vibration. — Impression  on  Bromide  of  Silver. — On  Chloride  of  Silver. — Total  Change  in  the  Distribution 
of  Heat  in  the  Interference  Spectrum  Page  52 

CHAPTER  VI. 

EXPERIMENTS   PROVING    THAT   IT  IS   IN    THE   YELLOW   REGION    OF  THE    SPECTRUM  THAT    THE  REDUCTION 
OF   CARBONIC  ACID   BY   THE   LEAVES  OF   PLANTS   TAKES  PLACE. 

Several  Imponderable  Principles  in  the  Sunbeam. — Sennebier's  Experiments  to  determine  to  which  of  these 
the  Effect  is  due. — Experiments  of  Morren  and  Daubeny. — Defects  of  the  Mode  of  operating  with  Absor- 
bent Media  and  Glasses. 

Decomposition  of  Carbonic  Acid  in  the  Prismatic  Spectrum.— Process  of  conducting  the  Experiment. — It  is 
in  the  Yellow  Region  that  the  Decomposition  takes  place. — No  Gas  is  evolved  in  the  Violet     .       .  61 

CHAPTER  VII. 

ON  THE   VARIOUS  IMPONDERABLE  AGENTS   EXISTING  IN   THE   DIFFERENT  REGIONS   OF   THE  SPECTRUM. 

Different  Agents  existing  in  the  Spectrum. — Description  of  the  Tithonic  Rays. — Their  Name. — Physical 
Independence  of  Heat. — Of  the  Chemical  Rays. — Their  Constant  Association  with  Light. — Detithonizing 
Action  of  Yellow  Sokitions. — Argument  for  their  Independence. — Other  Invisible  Principles  in  the  Sun- 
beam, such  as  the  Phosphoric  Rays. — Examination  of  the  Theory  of  M.  Becquerel  .        .        .  .65 

CHAPTER  VIII. 

IT  IS   YELLOW  LIGHT  WHICH   CONTROLS   THE   PROCESS   OF   DIGESTION  IN  PLANTS. 

Examination  as  to  which  of  the  Principles  mentioned  in  the  preceding  Chapter  is  engaged  in  the  Decompo- 
sition of  Carbonic  Acid. — It  is  not  Radiant  Heat. — Melloni  on  the  Ideal  Coloration  of  Heat. —  Analogies 
in  the  Case  of  Light. — Herschel's  Results. 

It  is  not  the  Tithonic  Ray. — Maximum  of  Decomposing  Action  for  Carbonic  Acid  and  Carbonaceous  Com- 
pounds, like  the  Retina,  is  in  the  Yellow  Ray. — Hence  the  Maximum  of  Visible  Illumination  coincides 
therewith    .................  72 

CHAPTER  IX. 

THEORY   OF   THE  ABSORPTION   OF   THE   TITHONIC   RAYS  AND  LIGHT. 

Estimate  of  the  Extent  and  Power  of  the  Solar  Radiations. — Influence  still  exists  in  the  Moonbeams. — Ab- 
sorptive Action  of  Chlorine  and  Hydrogen. — Detithonization  of  the  Ray  and  Tithonization  of  the  Gaseous 
Mixture. — Curve  and  Law. — Deductions  as  to  Latent  Light  and  Definite  Action. — Functions  discharged 
by  the  Chlorine  and  Hydrogen  respectively   ...........  76 

CHAPTER  X. 

THEORY   OF   IDEAL  COLORATION. 

Former  Observations  on  Colours  in  the  Chemical  Rays. — Nomenclature  derived  from  it. — Case  of  the  Chry- 
sotype. — Case  of  Bichromate  of  Potash. — Laws  deduced. — Control  of  Optical  Forces  over  Chemical  Ef- 
fects.— Application  to  Spectrum  Stains. — Herschel's  Law  for  Light. — Explanation  of  Variable  Effects  in 
Films  of  different  Thickness. — Mode  of  Action  of  the  Tithonic  Rays  .84 

CHAPTER  XI. 

ON   THE   MODE   OF  ACTION   OF  LIGHT   IN   DIRECTING   THE   DIGESTION   OF  PLANTS. 

Connexion  between  Absorption  and  Chemical  Action. — Radiant  Matter  is  absorbed  in  producing  different 
Effects. — Reappearance  of  the  Force  expended. — Laws  of  Preliminary  Absorption  and  Definite  Action 
observed  by  Plants. — Increased  Rapidity  of  Vegetation  implies  increased  Brilliancy  of  the  Incident  Light. 
— The  Sun  probably  a  Periodic  Star. — Description  of  the  Mode  of  Action  of  Light  and  Radiant  Heat  on 
Leaves        .................  91 

CHAPTER  XII. 

ON   THE   NERVOUS  AGENT  OF  PLANTS. 

Subdivisions  of  Nervous  Mechanism  in  Animals. — Excessive  Rapidity  of  Motion  arising  in  these  Nervous 
Actions. — Plants  constructed  on  a  Surface-type. —  Oxidating  Processes  replaced  in  them  by  the  Application 
of  Radiant  Heat. — Difference  of  Action  on  the  Upper  and  Under  Face  of  the  Leaf. — Light  applied  to  one, 
and  Heat  to  the  other  Face, — Specific  Effects  produced  by  the  different  coloured  Rays. — Eflects  of  these 
Radiant  Principles  on  the  Lower  Tribes  of  Animals.  — Centralization  of  Apparatus  for  different  Functions. 
—  Analogies  between  Nervous  Action  in  Animals  and  Imponderable  Agency  in  Plants.  — Vegetables  are 
the  Representatives  of  the  Resultant  Action  of  the  Ethereal  Agents  on  Ponderable  Matter. — Conclusion  99 


CONTENTS  OF  THE  APPENDIX. 


CHAPTER  I. 

EXPERIMENTS  MADE  TO  DETERMINE  WHETHER  LIGHT  EXHIBITS  ANY  MAGNETIC  ACTION. 

Character  of  the  Sky  in  Virginia. — Examination  of  Mr.  Christie's  Experiment. — Needles  not  affected  by  the 
Violet  Rays. — No  Reaction  between  a  Magnet  or  Voltaic  Currents  and  Light        .        .        .      Page  3 

CHAPTER  II. 

ON  THE  TIDAL  MOTIONS  OF  MOVABLE  ELECTRIC  CONDUCTORS. 

Description  of  the  Phenomenon. — The  Polar  Wires  act  as  Centres  of  Attraction.— They  produce  Tides. — 
Cause  of  the  Oscillations. — Cause  of  the  Spiral  Motions  8 

CHAPTER  III. 

ON  THE  INTERSTITIAL  MOVEMENTS  WHICH  TAKE  PLACE  AMONG  THE  PARTICLES  OF  BODIES. 

Of  the  Mode  of  Passage  of  Liquids  through  Pores. — Endosmosis. — Percolation  through  Gum  Lac,  Gold 
Leaf,  Mica,  etc. — Slow  Motions  in  the  Parts  of  Solid  Bodies,  as  in  Silver  Coins. — Percolation  through 
India  Rubber. —  Conditions  of  Equilibrium. — Percolation  through  Masses  of  Water. — Percolation  through 
excessively  thin  Films  of  Water,  as  Soap  Bubbles. — Analysis  of  Gas  on  the  Exterior  and  in  the  Interior 
of  the  Soap  Bubble. — General  Law  of  the  Phenomenon  deduced    .......  13 

CHAPTER  IV. 

ON  INTERSTITIAL  MOVEMENTS,  being  a  Continuation  of  the  preceding  Chapter. 
Diffusion  takes  place  between  the  Particles  of  Heterogeneous  Bodies. — Differs  from  Chemical  Attraction. — 
Action  of  Binary  Arrangements. — Action  of  Ternary  Arrangements. — Decompositions  by  Binary  Arrange- 
ments.— Decompositions  by  Ternary  Arrangements       .........  23 

CHAPTER  V. 

THE  PHYSICAL  THEORY  OF  CAPILLARY  ATTRACTION. 

Importance  of  Capillary  Attraction  in  Physiology. — Capillary  Attraction  is  an  Electrical  Phenomenon. — Its 
Physical  Theory. — The  Effect  varies  with  Variations  of  Electric  Disturbance. — Takes  place  between 
Bodies  of  different  Forms. — Physiological  Illustrations  .        ........  34 

CHAPTER  VI. 

ON  THE  GREAT  MECHANICAL  FORCES  GENERATED  BY  THE  CAPILLARY  ATTRACTION  OF  CELLULAR  TISSUE. 

Physiological  Relations  of  Cellular  Tissue. — Force  with  which  Gases  and  Liquids  pass  through  Cellular 
Tissue. — Disturbing  Action  of  Leakage. — The  Capillary  Force  overcomes  powerful  Mercurial  Pressure. 
— Dalton's  Hypothesis. — The  Tissue  is  the  Origin  of  the  Force. — Its  Absorbent  and  Condensing  Action. 
— Voltaic  Batteries  may  be  used  for  producing  great  Pressures. — Gases  pass  when  resisted  by  the  Force 
of  many  Atmospheres. — The  condensed  Gas  acts  as  a  Vacuum. — Co-ordination  of  the  Results  of  Dalton, 
Graham,  and  Mitchell. — Disturbing  Agencies. — Disturbance  by  Variation  of  Temperature. — Physiological 
Experiments  and  Remarks  ..............  42 

CHAPTER  VII. 

THE  PHYSICAL  THEORY  OF  ENDOSMOSIS. 

Relation  of  Endosmosis  to  Capillary  Attraction. — Cases  of  reported  Decompositions. — Can  be  produced  by 
Inorganic  Masses,  and  therefore  not  due  to  Vitality. — Water  made  to  wet  Mercury. — Voltaic  Battery  con- 
trols Capillary  Attraction. — Action  of  Inorganic  Tissues. — Water  passes  through  excessively  small  Pores. 
— Hydraulic  Currents. — Deposites  produced  by  Endosmotic  Currents. — Apparent  Decomposition  of  Metal- 
lic Salts  by  Membranes. — True  Theory  of  it. — General  Conclusion  that  Endosmosis  is  nothing  more  than 
common  Capillary  Attraction,  and  never  occasions  true  Decompositions  ......  57 

CHAPTER  VIII. 

ON  THE  USE  OF  A  SECONDARY  WIRE  AS  A  MEASURE  OF  THE  RELATIVE  TENSION  OF  ELECTRIC  CURRENTS. 

Object  of  the  Memoir. — Action  of  a  Secondary  Wire. —  Description  of  the  Torsion  Galvanometer. — Resist- 
ance of  the  Secondary  Wire  under  Variations  of  Tension. — Condition  of  the  Current  never  changes. — 
Tension  rises  with  Length  of  Wire  and  with  Distance  of  Plates. — Relation  between  Quantity  and  Tension. 
— Theory  of  Tension  of  the  Voltaic  Battery. — Known  Methods  of  increasing  Tension  of  Currents.— General 
Law. — Case  of  Thermo-Electricity  and  Machine  Electricity. — Voltaic  Spark  before  Contact  in  Vacuo  73 


X 


CONTENTN  Ob'  THIO  APPENDIX. 


CHAPTER  IX. 

ON  THE   ELECTROMOTIVE   POWER  OF  HEAT. 

Object  of  the  Memoir. — Experimental  Arrangement  to  determine  the  Electromotive  Power. — Temperatures 
calculated  from  Quantities  of  Electricity. — Increase  of  Tension  with  Increase  of  Temperature. — Depends 
on  increased  Resistance  to  Conduction. — Quantity  of  Electricity  independent  of  heated  Surface. — In  Ther- 
mo-electric Piles,  the  Quantity  of  Electricity  proportional  to  the  Number  of  Pairs. — Best  Forms  of  Con- 
structioa  of  Thermo-electric  Pairs       ...........    Page  90 

CHAPTER  X. 

EXPERIMENTS  ON   THE  CHEMICAL  ACTION  OF  SOLAR  LIGHT. 

Action  of  Absorbent  Media. — Ideal  Coloration  of  the  Chemical  Rays. — Specific  Absorption. — Colorific  Ab- 
sorption.— Calorific  Absorption. — Specific  Absorption  of  the  Chemical  Rays. — Effect  of  Yellow  Solutions. 

Decomposition  of  Carbonic  Acid  by  Leaves. — Penetration  of  Dimensions  in  Giases. — Decomposiiion  of  Car- 
bonic Acid  under  various  coloured  Media. —  Gas  from  Leaves  contains  Nitrogen. — Chemical  Rays  of  dif- 
ferent Colours. — Identity  of  Volume  in  the  absorbed  and  evolved  Gas.  —  Cause  of  the  Decomposition. 

Ritter's  Experiments  of  the  Non-oxygenation  of  Phosphorus. 

Decomposition  of  the  Salts  of  Silver. — Prismatic  Spectrum  on  Bromide  of  Silver. — Interference  of  Chemical 

Rays. — Salts  decomposed  by  Light. — Moonlight  and  Artificial  Flames  are  inactive. 
Of  Perihelion  Motions. — Dew  of  Water  and  Mercury. — Iodine. — Chloride  of  Gold. — Non-deposition  on  a 

Glass  Plate. — Current  Action. — Action  of  Flame. — Action  of  Metal  Screens. — Protecting  i\ction  of  a 

Metal  Ring. — Is  there  Electricity  in  the  Solar  Ray  ? 
Cause  of  the  Green  Colour  of  Leaves. — Plants  grow  in  Lights  of  various  Colours. — Seeds  also  germinate  in 

Red,  Yellow,  and  Blue  Light. — Chemical  Rays  of  different  Colours      .        .        ....  99 

Note  added  to  the  foregoing  Chapter. 

AN   ACCOUNT  OF   SOBIE   EXPERIMENTS  MADE  IN  THE   SOUTH   OF  VIRGINIA  ON   THE   LIGHT  OF   THE  SUN  133 

CHAPTER  XI.' 

ON  THE  PROCESS  OF  DAGUERREOTYPE,  AND  ITS  APPLICATION  TO  TAKING  PORTRAITS  FROM  THE  LIFE. 

Daguerreotype  Portraits  from  the  Life  first  taken. — Spectral  Images.— Preservation  of  the  Sensitive  Plate 
increases  the  Sensitiveness. — Modifications  in  the  Daguerreotype  Process. — Moonlight,  Artificial  Light, 
and  Drummond's  Light,  are  all  Active. — Description  of  the  original  Process  of  taking  Portraits  from  the 
Life  136 

CHAPTER  XII. 

ON  SOME  ANALOGIES  BETWEEN  THE  PHENOMENA  OF  THE  CHEMICAL  RAYS  AND  THOSE  OF  RADIANT  HEAT. 

Object  of  the  Memoir. — On  the  Daguerreotype  Process. — Chemical  Constitution  of  Daguerreotype  Pictures. 
— Spectral  Images. — Film  of  Iodide  torn  off  Mechanically. — Iodine  is  not  evolved,  but  corrodes  the  Plate. 
— The  Chemical  Rays  are  absorbed. — The  Photographic  Effects  are  transient. — The  Chemical  Rays  are 
not  conducted. — They  become  Latent. — Optical  Qualities  control  Chemical  Action. — The  Active  Rays 
are  absorbed,  and  the  Complementary  reflected. — Relation  of  Optical  Forces  and  Chemical  Affinities  144 

Note  added  to  the  preceding  Chapter. 

ON  CERTAIN  SPECTRAL  APPEARANCES,  AND  ON  THE  DISCOVERY  OF  LATENT  LIGHT       .  .  156 

CHAPTER  XIII. 

ON   A  NEW   IMPONDERABLE   SUBSTANCE,  AND   ON   A    CLASS   OF    CHEMICAL  RAYS   ANALOGOUS   TO  THE   RAYS  OF 

DARK  HEAT. 

Analogies  between  the  Cheinical  Rays  and  Heat. — New  Nomenclature  proposed. — Tithonic  Rays. — In- 
dependence of  Tithonic  Rays  and  Light. — Independence  of  Tithonic  Rays  and  Heat. — Dark  Tithonic 
Rays        .        .        .        ."  158 

Note  added  to  the  preceding  Chapter. 

ON  THE  RAPID  DETITHONIZING  POWER  OF  CERTAIN  GASES  AND  VAPOURS,  AND  ON  AN  INSTANTANEOUS  MEANS 

OF  PRODUCING  SPECTRAL  APPEARANCES  ......  165 

CHAPTER  XIV. 

ON  A   NEW  SYSTEM   OF  INACTIVE  TITHONOGRAPHIC  SPACES  IN   THE  SOLAR  SPECTRUM  ANALOGOUS  TO  THE  FIXED 

LINES  OF   FRAUNHOFER,   AND   ON   THE  TITHONOTYPE. 

Mode  of  Producing  the  fixed  Lines. — Description  of  them. — Difficulty  of  obtaining  them  in  the  Yellow  and 
Green. 

Daguerreotypes  are  dotted  Surfaces. — Mode  of  copying  them  by  the  Tithonotype.— Polarized  Structure  of 
the  Dagucrreoivpp  Film      .............  169 


CONTENTS  OF  THE  APPENDIX. 


xi 


CHAPTER  XV. 

ON  THE  DECOMPOSITION  OF  CARBONIC  ACID  GAS  AND  THE  ALKALINE  CARBONATES  BY  THE  LIGHT  OF  THE  SUN, 

AND   ON  THE  TITHONOTYPE. 

Dr.  Daubetiy's  Experiments. — Importance  of  the  Subject. — Decomposition  in  the  Prismatic  Spectrum. — De- 
composition under  Absorbent  Media. — Decomposition  is  due  to  Light. — Disturbing  Causes. — Analysis  of 
Gas  evolved. — Decomposition  of  Saline  Bodies. — Production  of  Nitrogen. — Disappearance  of  Oxygen. — 
Character  of  Chlorophyl. 

Tithonotypes  in  Copper. — Detithonizing  Power  of  Gases  ........  Page  175 

CHAPTER  XVI. 

DESCRIPTION  OF  THE  TITHONOMETER,  AN  INSTRUMENT  FOR  MEASURING  THE  CHEMICAL  FORCE  OF  THE  INDIGO- 

TITHONIC  RAYS. 

The  Instrument  consists  of  a  Mixture  of  Chlorine  and  Hydrogen. — It  is  acted  upon  by  Lamp  Light,  an  Elec- 
tric Spark  at  a  Distance,  &c. — Chlorine  and  Hydrogen  unite  in  Proportion  to  the  Amount  of  Light. — Mode 
of  measuring  out  known  Quantities  of  Rays. — The  Maximum  of  Action  is  in  the  Indigo  Space.  — Construc- 
tion of  the  Instrument. — Theoretical  Conditions  of  Equilibrium. — Preliminary  Adjustment. — Method  of 
continuous  Observation. — Method  of  interrupted  Observation. — Remarkable  Contraction  and  Expansion  187 

CHAPTER  XVII. 

ON  TITHONIZED  CHLORINE. 

Description  of  the  Experiment. — The  Change  in  the  Chlorine  is  not  Transient. — There  are  two  Stages  in 
the  Phenomenon. — Rays  are  absorbed  in  producing  this  Change. — It  is  the  Indigo  Ray  which  is  absorbed. 
— The  Action  is  positive  from  End  to  End  of  the  Spectrum. — The  Indigo  Ray  forms  Muriatic  Acid,  as 
well  as  produces  the  Preliminary  Tithonization. — Change  in  other  Elementary  Bodies. — Verification  of 
the  preceding  Results  with  the  Tithonometer  198 

CHAPTER  XVIII. 

FARTHER  CONSIDERATIONS  ON  THE  EXISTENCE  OF  A  FOURTH  IMPONDERABLE. 

Defects  of  former  Evidence. — Anew  Photometer. — Measures  of  the  Light  transmitted  by  Coloured  Solutions. 

— Explosion  of  Chlorine  and  Hydrogen  by  a  distant  Electric  Spark. — Absorptive  Action  of  Media. 
The  Absorptive  Action  on  Light  and  the  Tiihonic  Rays  follows  different  Laws. 

Opacity  of  Glass  for  Phosphoric  Rays. — Determination  of  the  Refrangibility  of  the  Phosphoric  Rays  of  an 
Electric  Spark. — Refrangibility  of  the  same  Rays  in  the  Voltaic  Arc  of  Flame. — Professor  Henry's  Ex- 
periments. 

These  Facts  serve  to  prove  that  there  are  more  than  three  Imponderables  20.5 


I 


I 


INTRODUCTION. 


GENERAL    REMARKS    ON    THE    INFLUENCE    OF    PHYSICAL    AGENTS    ON  ORGANIZATION 

AND  LIFE. 

Contents  :  Object  of  the  Work  stated. — Connexion  of  Organization  icith  the  Imponder- 
able Principles. — General  Laws  direct  all  Astrononiical  Occurrences  and  Physical 
Events. —  Transitory  Nature  of  all  Combinations,  and  especially  those  wltick  are  Or- 
ganized.—  Time  is  an  Element  of  Life. — Inorganic  Changes  are  brought  about  by 
Physical  Laics,  as  also  is  the  E.rfinction  of  Living  Paces. — Influence  of  Climate  on 
the  Distribution  and  Character  of  Animals  and  Plants. — Relations  between  Ariimated 
Forms  and  the  Atmosphere. — Lrfluence  of  Currents  in  the  Air  and  in  the  Sea. — 
Gradual  Emancipation  of  the  Higher  Races  from  the  Direct  Action  of  External 
Agents. 

1.  The  rapid  progress  of  organic  chemistry  has  recently  drawn  the  attention  of  sci- 
entific men  to  many  remarkable  relations  which  exist  between  animated  beings  and  the 
inorganic  world.  For  a  long  period,  physiological  doctrines,  the  spirit  of  which,  for 
many  ages,  has  undergone  but  little  change,  offered  an  insurmountable  barrier  to  the 
application  of  methods  of  physical  research  to  problems  connected  with  the  phenome- 
na of  life.  On  these,  in  our  times,  a  successful  inroad  has  been  made,  chiefly  through 
the  aid  of  improved  methods  of  chemical  analysis.  In  a  philosophical  point  of  view, 
it  was  the  office  of  the  seventeenth  century  to  unfold  the  doctrine  of  universal  gravita- 
tion, to  assign  proper  causes  for  the  motions  of  the  celestial  bodies,  and  to  develop  the 
great  doctrines  of  astronomy.  It  was  the  office  of  the  eighteenth  to  lay  the  founda- 
tions of  physics  and  chemistry,  or  of  that  group  of  sciences  which  embraces  the  rela- 
tions and  reactions  of  atoms.  It  is  the  office  of  the  nineteenth  to  discover  the  laws 
which  obtain  in  the  complicated  structure  of  animated  beings,  those  laws  which  give 
rise  to  the  mysterious  phenomena  which  we  call  life. 

2.  This  book,  in  which  will  be  found  some  facts  which  it  has  happened  to  its  author 
to  discover,  is  offered  as  an  humble  contribution  among  those  more  brilliant  gifts  with 
which  Germany,  and  France,  and  England  have  lately  enriched  vegetable  and  animal 
physiology.  It  treats  of  a  subject  which  forms  the  connecting  link  between  pure  chem- 
istry and  those  higher  sciences.    The  great  idea  which  it  is  designed  to  illustrate  is 

A 


2 


CONNEXION  OF  ORGANIZATION  WITH  THE  IMPONDERABLES. 


that  which  connects  the  production  and  phenomena  of  organized  beings  with  the  im- 
ponderable principles. 

3.  In  this  work  the  existence  of  the  Vital  Force  of  physiologists — as  a  homogeneous 
and  separate  force — is  uniformly  denied.  The  progress  of  science  shows  plainly  that 
hving  structures,  far  from  being  the  product  of  one  such  homogeneous  power,  are  rather 
the  resultants  of  the  action  of  a  multitude  of  natural  forces.  Gravity,  cohesion,  elasti- 
city, the  agency  of  the  imponderables,  and  all  other  powers  w^hich  operate  both  on 
masses  and  atoms,  are  called  into  action,  and  hence  it  is  that  the  very  evolution  of  a 
living  form  depends  on  the  condition  that  all  these  various  agents  conspire.  There  is 
no  mystery  in  animated  beings  which  time  will  not  at  last  reveal.  It  is  astonishing 
that,  in  our  days,  the  ancient  system,  which  excludes  all  connexion  with  natural  phi- 
losophy and  chemistry,  and  depends  on  the  fictitious  aid  of  a  visionary  force,  should 
continue  to  exist :  a  system  which,  at  the  outset,  ought  to  have  been  broken  down  by 
the  most  common  considerations,  such  as  those  connected  with  the  mechanical  princi- 
ples involved  in  the  bony  skeleton,  the  optical  principles  in  the  construction  of  the  eye, 
or  the  hvdraulic  action  of  the  valves  of  the  heart. 

4.  In  their  origin,  all  those  important  ideas  which  now  constitute  modern  science 
have  been  obscurely  and  imperfectly  set  forth.  It  is  not  given  to  the  human  mind, 
when  it  emerges  from  the  darkness  of  ignorance,  any  more  than  to  the  human  eye, 
when  it  emerges  from  physical  darkness  into  the  sunshine,  to  see  all  objects  which  are 
before  it  in  their  proper  aspect  and  position.  A  period  of  time  must  elapse,  during 
which  we  become  accustomed  to  the  light.  Future  discovery,  in  its  progress,  may 
show  that,  of  the  facts  brought  forward  in  this  volume,  many  are  misplaced,  and  many 
misapplied ;  these  are  incidents  to  which  all  philosophical  works  are  liable.  But  if  it 
should  happen  that  anything  contained  herein  shall  aid  in  fastening  the  attention  of 
men  of  science  on  the  idea  wdiich  it  is  designed  to  impart,  the  author  will  have  received 
his  reward,  and  the  labours  of  ten  years  will  not  have  been  entirely  thrown  away. 

5.  Organized  beings  and  organized  bodies  spring  forth  in  those  positions  only  to 
which  the  rays  of  the  sun  have  access.  They  are,  therefore,  limited  to  the  atmosphere, 
the  sea,  and  the  surface  of  the  earth.  Periodical  vicissitudes,  which  are  observed  both 
in  vegetables  and  in  animals,  serve  to  show  that  this  is  not  a  mere  fortuitous  coinci- 
dence, but  rather  an  intimate  connexion  between  the  phenomena  of  life  and  the  pres- 
ence of  the  imponderables.  When  the  sun  is  set,  the  leaves  of  plants  no  longer  decom- 
pose the  carbonic  acid  of  the  air,  but  a  pause  takes  place  in  the  activity  of  their  func- 
tions, and  they  sink  into  a  passive  condition.  The  gaseous  bodies  brought  from  the 
ground  by  the  action  of  the  spongioles  percolate  through  the  delicate  tissues  of  the  leaf, 
and  escape  away  into  the  atmosphere.  At  night,  also,  in  many  flowers,  the  petals  fold 
themselves  together,  and,  for  a  time,  all  active  processes  cease.  It  is,  therefore,  through 
an  instinctive  impulse,  that  comes  over  them  during  this  period,  that  all  animals,  except 
such  as  take  their  prey  by  night,  seek  places  of  rest.  Darkness,  and  silence,  and  re- 
pose are  all  connected  together. 

0.  It  is  one  of  the  greatest  discoveries  of  the  present  age,  that  the  races  of  animals 
which  have  inhabited  our  globe  were  of  successive  creation ;  that  they  constitute  a 


GENERAL  LAWS  DIRECT  ALL  EVENTS.  3 

series,  the  extreme  terms  of  which  bear  no  resemblance  to  each  other;  that,  commen- 
cing with  those  of  the  earUest  date,  we  are  able  to  trace  a  constant  progress  both  in 
intellectual  and  structural  development.  In  all,  there  are  found  evidences  of  the  opera- 
tion of  the  same  formative  power,  acting  by  and  resorting  to  the  aid  of  the  same  phys- 
ical principles.  The  trilobite,  of  the  primary  fossiliferous  rocks,  and  man,  of  the  most 
recent,  have,  at  the  first  glance,  little  in  common,  but  a  more  attentive  observation  soon 
shows  that  the  idea  of  construction  in  each  is  so  allied,  that  they  have  both  undoubt- 
edly sprung  from  the  operations  of  the  same  Intelligent  Mind. 

7.  Do,  then,  these  successive  races  of  sentient  beings  form  altogether  a  strictly  con- 
tinuous series  ?  As  in  the  series  of  mathematicians,  where  each  term  bears  a  definite 
relation  to  those  which  precede  it,  and  contains  within  itself  the  elemental  law  of  those 
which  are  to  come  after  it,  do  each  one  of  these  organized  beings  observe  a  position  of 
relationship  with  those  that  are  of  earlier,  and  those  also  which  are  of  later  date  ?  Do 
the  animals  and  the  plants  of  the  Carboniferous  Period  connect  those  of  the  Silurian 
with  those  of  the  Newer  Pleiocene  ?  Were  the  organized  mechanisms  of  the  Old  Red 
Sandstone  essential  to  the  appearance  and  existence  of  those  which  live  with  us  ?  Are 
we,  in  short,  to  regard  the  Author  of  these  wonderful  forms  as  operating  in  each  one 
of  these  instances  by  the  same  law,  and,  from  small  beginnings,  evolving  and  transform- 
ing the  most  elaborate  by  a  successive  passage  through  those  which  are  inferior  1  or 
are  we  to  understand  that,  at  particular  and  unconnected  epochs,  the  broad  hand  of  an 
overruling  Providence  is  to  be  discovered,  fashioning  and  framing  each  class  of  created 
forms,  irrespective  of  external  physical  forces  or  agents,  and  giving  birth  spontaneously 
to  unconnected  tribes  of  animals  and  plants,  which  bear  n«  sort  of  relationship  to  one 
another,  and  are  not  parts  of  one  common  plan  in  which  there  is  a  unity  of  design  ? 

8.  The  interior  movements  of  the  solar  system,  and  the  collapsing  of  nebular  masses, 
are  committed  to  secondary  agents  or  to  inmiutable  laws ;  and  these  are  events  which, 
for  their  completion,  often  require  great  periods  of  time.  No  invisible  or  extraneous 
agency  ever  intervenes.  In  her  predestined  course,  the  moon  revolves  and  exhibits 
her  phases,  and,  like  the  beating  pendulum,  though  ten  thousand  years  may  have  elap- 
sed since  its  last  beat,  the  comet — the  pendulum  of  the  universe — swings  punctually  past 
the  sun.  That  universe  is  not  an  unchangeable  mass,  but  is  made  up  of  moving  and 
revolving  orbs,  which  are  all  obedient  to  one  common  law.  Do  not,  therefore,  such 
things  point  out,  that,  in  the  midst  of  all  these  transitory  affairs,  immutable  principles 
are  involved,  and  a  common  law  is  incessantly  in  operation  ? 

9.  In  the  history  of  the  human  race,  it  may  be  observed  that  epochs  have  occurred, 
which,  following  each  other  with  a  kind  of  periodicity,  have  stood  in  relationship  with, 
or  even  brought  about,  the  conditions  of  modern  civilization.  As  in  the  course  of  the 
life  of  an  individual  there  is  no  incident  which  is  not  in  connexion  with  circumstances 
which  have  preceded  it,  and  none  that  does  not  give  a  bent  to  those  that  follow,  we 
naturally  view  the  vicissitudes  of  our  existence  as  bearing  the  relation  of  cause  and 
effect.  We  trace  the  circumstances  of  to-day  from  the  circumstances  of  yesterday. 
In  the  movements  of  the  celestial  bodies  we  continually  see  fixed  events  resulting  from 
the  operation  of  apparently  variable  causes;  the  waxing  and  waning  of  the  moon,  the 


4 


TRANSITORY  NATUKK  OF  ORGANIZED  COMBINATIONS. 


eclipses  of  the  sun,  the  transitory  visits  of  comets.  On  our  own  earth  we  also  witness 
oceanic  tides  which  ebb  and  flow,  and  spring  and  summer,  and  autumn  and  winter, 
following  each  other  unceasingly.  From  year  to  year  we  witness  the  alternate  in- 
crease and  shortening  of  the  day,  the  tempests  of  the  equinoxes,  and  the  sultry  weather 
of  midsummer.  In  the  world  of  organization  which  is  around  us,  we  observe  similar 
mutations  :  there  are  plants  which  come  up  in  spring,  and  die  away  in  autumn  ;  and 
even  those  hardier  races  which  witness  the  changing  of  empires,  give  tokens  that  they 
are  included  within  this  law  of  variation.  The  oak  unfolds  its  buds  into  leaves,  which 
periodically  fall  to  the  ground.  Among  sensitive  beings,  from  time  to  time,  different 
races  of  animals  have  held  dominion  of  the  earth  :  at  one  period  it  has  been  almost  the 
exclusive  abode  of  reptiles,  at  another  of  four-footed  mammalians,  and  at  last  is  under 
the  control  of  two-handed  man. 

10.  In  whatever  direction,  therefore,  we  look,  we  perceive  the  transitory  nature  of 
all  things :  even  with  those  which,  from  their  magnitude,  their  remoteness,  or  their  im- 
portance, might  with  apparent  propriety  be  regarded  as  not  participating  in  these  un- 
ceasing changes,  the  law  holds  good.  Throughout  the  universe  there  is  no  monument 
that  retains  its  primordial  condition. 

11.  Each  one  of  these  various  changes,  no  matter  whether  it  concerns  organic  or 
inorganic  nature,  has  been  the  result  of  the  action  of  some  determining  cause.  The 
countless  systems  of  phenomena  which  have  arisen  are  all  connected  together  as  sys- 
tems of  effects.  In  a  web,  as  it  passes  from  the  loom,  the  different  threads  interlace 
with  one  another,  and  though  we  soon  cease  to  identify  each  as  it  pursues  its  sinuous 
way,  we  know  that  the  last  is  connected  with  the  first;  and  in  the  web  of  nature  each 
event  has  been  brought  into  relation  with  others  that  have  gone  before  it,  and  others 
that  have  succeeded  it,  and  all  are  intertwined  together  as  a  series  of  causes  and  effects. 

12.  Understanding  thus  that  no  effect  takes  place  except  by  the  operation  of  some 
prior  cause,  we  occupy  ourselves  in  discovering  its  dependances  on  things  that  have 
preceded  it.  It  is  this  which  engages  the  contemplation  of  the  greatest  philosophers  in 
their  speculations  on  natural  phenomena,  and  ordinary  men  in  the  daily  affairs  of  life. 
The  disasters  of  to-day  we  attribute  to  the  errors  of  yesterday,  and  the  possession  of 
glory,  and  wealth,  and  position  in  society,  to  plans  that  have  been  conducted  well.  No 
man  is  in  heart  a  fatalist,  for  each  one  clearly  perceives  that  his  destinies  are  in  his 
own  charge  ;  and  no  small  portion  of  human  happiness  or  misery  springs  from  a  knowl- 
edge of  these  things.  In  other  tribes  of  life,  where  intellectual  processes  are  replaced 
by  processes  of  instinct,  it  is  very  different :  the  wild  animal  which  hves  in  the  prairies 
meets  seasons  of  famine  and  distress  without  a  moan.  Careless  for  to-morrow,  he 
bears  up  with  his  present  lot,  and  is  utterly  unconscious  that  he  may  have  been  the 
author  of  his  own  wo.  There  is  with  him  no  recording  memory  of  whence  he  came, 
no  distracting  care  of  whither  he  may  go.  He  acts,  in  respect  of  the  passing  time,  in 
the  same  way  that  men  act  in  respect  of  their  whole  hfe ;  they  are  ignorant  of  what 
happened  in  their  early  days  of  infancy,  and  never  trouble  themselves  about  what  may 
come  in  old  age. 

13.  The  vegetable  world,  from  possessing  no  nervous  system,  is  inherently  incapable 


RELATION  OF  ANIMALS  TO  MAN. 


5 


of  appreciating  its  own  existence,  and  mucli  more,  therefore,  of  feeling  emotions  of 
pleasure  and  pain.  To  animals  nature  has  given  a  present  contentment,  which  is  pur- 
chased by  ignorance ;  but  to  men,  who  are  endowed  with  reasoning  powers,  whose 
nervous  system  has  been  so  formed  as  to  enable  their  mental  operations,  by  processes 
of  memory,  to  reach  over  long  periods  of  time,  to  combine  together  events  which  are 
afar  off,  to  decompose  into  their  constituent  parts  phenomena  that  are  complex,  and  to 
trace  each  one  of  those  parts  up  to  its  proper  cause,  knowledge  has  been  given,  and 
the  price  of  that  knowledge  is  pain. 

14.  So  far  as  their  intellectual  powers  are  concerned,  the  life  of  animals,  even  of 
those  of  the  highest  orders,  is  analogous  to  the  dreamy  sleep  of  man.  Overcome  by 
the  toils  of  the  day,  the  weary  labourer  sinks  into  repose,  and  there  come  before  him 
pageants  and  scenery  connected,  to  a  certain  extent,  with  the  external  world  ;  but  of 
that  world  he  is  wholly  unconscious.  Instead  of  the  accustomed  forms  that  he  meets 
in  his  daily  affairs,  there  spring  up  light  aerial  shapes  and  phantasms.  From  recesses 
in  the  brain,  where  they  have  been  long  stored,  and  perhaps  forgotten,  the  recol- 
lection of  landscapes  that  he  has  seen  of  old  come  forth ;  he  views  the  well-known 
forms  unfold  themselves  before  him  :  there  stands  the  aged  oak,  at  the  foot  of  which  he 
has  so  often  watched  the  setting  of  the  sun,  and  there  is  the  pale-blue  sky  with  its 
gilded  clouds,  and  in  the  distance  the  almost  invisible  mountains.  That  fairy  panorama 
has  its  shadowy  tenants,  which  live,  and  move,  and  breathe  :  the  dead  are  also  there. 
From  those  silent  sepulchres  which  are  within  the  brain,  they  rise  again  as  living 
things,  and  people  the  scenes  they  once  loved.  They  converse  with  and  counsel  the 
dreamer. 

15.  As  thus,  during  the  night,  these  phantoms  come  up  spontaneously  before  us,  and 
spontaneously  disappear,  and  time,  of  which  men  gain  a  knowledge  only  by  comparing 
events,  passes  unnoticed  away,  we  see  to  what  a  small  extent  the  will  controls  these 
phenomena.  The  spectres  come  unbidden,  and  they  as  suddenly  depart,  and  very 
often,  during  one  slumber,  they  change  and  rechange  again,  and  memory,  the  arch- 
conjuror,  evokes  scene  after  scene. 

16.  With  the  brute  creation  the  same  thing  holds.  In  their  daily  relations  with  ex- 
ternal nature,  almost  all  their  functions  are  carried  on  by  the  promptings  of  instinct,  or 
in  a  mechanical  way.  So  far  as  we  can  see,  the  current  of  thought  seems  to  be  little 
under  their  control,  and  they  have  no  power  of  effecting  the  collocation  of  ideas.  They 
cannot  tell  the  passing  of  time,  which  flows  away  in  its  silent  lapse,  and  leaves  them, 
as  it  found  them,  contented.  Unaided  by  the  instincts  which  are  implanted  in  them, 
they  show  but  little  power  of  reasoning  from  causes  to  effects,  or  from  effects  to  causes. 
Even  in  the  most  obvious  cases,  where  actions  are  performed  before  them  which,  if 
performed  by  themselves,  might  tend  greatly  to  their  enjoyment,  they  exhibit  but  a  low 
imitative  power.  Though  he  has  often  seen  it  done  by  man,  the  monkey  has  never 
yet  learned  to  make  a  fire. 

17.  All  objects  which  surround  us,  whether  animate  or  inanimate,  are  marked  by  a 
transitory  nature.  They  come  into  existence,  for  a  while  they  continue,  and  then  they 
pass  away.    It  is  thus  that,  in  the  course  of  ages,  the  configuration  of  continents  and 


6 


TIME  IS  AN  ELEMENT  OF  LIFE. 


seas  undergoes  change — changes,  however,  which  require  long  periods  of  time,  and 
which,  in  the  course  of  human  existence,  are,  for  the  most  part,  imperceptible.  Com- 
mencing with  the  inorganic  world,  which  we  see  is  included  in  this  law  of  unceasing 
variation,  we  discover  that  each  of  its  component  structures  passes  through  its  trans- 
mutations more  rapidly  according  as  its  constitution  is  more  complicated.  For  the 
same  reason  it  is  that  bodies  which  are  organized — and  organization  in  itself  implies 
complicated  structure — are  of  all  forms  most  liable  to  these  mutations.  The  dead  car- 
cass of  an  animal  speedily  disappears  under  the  forms  of  water,  anmionia,  and  carbonic 
acid,  its  elemental  atoms  breaking  up  into  simpler  and  more  enduring  groups.  It  is  a 
vulgar  error  that  a  living  being  possesses  a  principle  of  resistance  to  external  agents, 
while  a  dead  one  submits  itself  to  them  ;  both  equally  change,  or,  of  the  two,  the  living 
one  putrefies  and  changes  the  more  rapidly ;  but,  then,  for  each  of  the  several  systems 
of  dead  and  dissevered  atoms,  appointed  routes  of  passage  are  prepared.  The  carbonic 
acid  escapes  by  the  lungs,  the  nitrogenized  compounds  through  the  kidneys,  and  water 
through  both  those  organs  and  the  skin.  The  putrefaction  of  an  organized  being  is  a 
constant  event ;  it  commences  before  birth,  and  continues  after  death ;  but,  though 
constant,  it  is  regulated  in  life,  and  after  death  the  avenues  of  discharge  are  all  closed, 
and  the  mechanism  appointed  for  removing  the  decaying  atoms  broken  down. 

18.  Time,  therefore,  enters  as  an  element  in  animal  life.  Individuals,  after  the  prog- 
ress of  a  few  years,  pass  away,  and,  during  each  moment  of  their  existence,  their  va- 
rious parts  are  undergoing  incessant  change.  There  is  a  constant  removal  of  all  the 
carbon  compounds  from  every  part  of  the  system  ;  a  removal  which  necessarily  arises 
in  conducting  locomotion,  and  various  other  functions.  If  an  electric  current  is  to  be 
passed  along  the  wire  of  a  voltaic  battery,  and  is  required  to  evolve  a  certain  amount 
of  hght  or  heat,  or  to  produce  a  certain  amount  of  electro-magnetic  motion,  a  fixed 
amount  of  zinc  must  be  consumed.  If  a  steam  engine  has  a  given  quantity  of  work  to 
perform,  a  given  quantity  of  coal  must  be  burned.  So  also  in  animal  systems,  the  pro- 
duction of  motion  can  only  be  effected  by  the  consumption  of  the  parts  of  the  animal 
machine.  In  the  higher  races,  in  which  an  elaborate  development  of  these  functions 
has  been  accomplished,  the  processes  of  transmutation  go  on  with  the  greatest  rapidity. 
Among  insects,  which  are  constantly  upon  the  wdng,  the  combustion  of  the  organic 
atoms  is  at  a  maxinmm,  as  is  also,  consequently,  the  production  of  heat ;  but  at  night, 
or  when  they  rest,  the  rate  of  respiration  diminishes,  the  heat  decUnes,  and  the  trans- 
mutation is  checked.  In  the  existence  of  an  animal,  as  also  in  the  existence  of  its 
constituent  atoms,  time,  therefore,  enters  as  an  element. 

19.  There  is  a  constant  washing  away  of  mountains  into  the  sea,  and  rivers  contin- 
ually tend  to  fill  up  their  beds.  In  one  region  the  detritus  of  the  land,  brought  down 
by  streams,  encroaches  on  the  ocean,  and  makes  new  countries ;  in  another,  the  ocean 
invades  the  shores,  and  makes  changes  in  the  shape  of  continents.  In  the  vegetable 
world,  the  leaves  are  organized  in  spring,  and  decay  in  autumn  ;  and  among  animals, 
each  has  its  own  period  of  duration.  Among  all  these  organized  structures  the  parts 
are  undergoing  unceasing  change.  Physical  forces  are  at  work,  physical  phenomena 
have  to  be  originated,  and  physical  ends  to  be  gained.    The  death,  therefore,  of  an  or- 


LAW  OF  CHANGE  IN  ATOMS,  INDIVIDUALS,  AND  RACES.  7 

ganic  atom  in  an  animal,  has  for  its  object  tlie  production  of  a  given  result,  and  is 
itself  the  result  of  the  action  of  ordinary  physical  powers. 

20.  If,  thus,  the  various  movements  I  am  executing  in  writing  these  pages  arise  di- 
rectly from  the  respiration  of  oxygen  gas,  and  its  transmission,  by  arterial  blood,  through 
the  system,  and  each  letter  that  I  have  penned  is  the  result  of  the  death  of  parts  of  this 
animal  frame,  w^hich,  in  their  removal,  have  ended  in  the  production  of  motion  and 
heat,  it  is  plain  that  there  is  no  essential  difference  between  the  death  of  an  organic 
atom  and  the  extinction  of  an  animal  race.  Of  the  thousands  of  animal  forms  which 
have  ceased  to  live,  and  of  which,  indeed,  we  should  have  no  knowledge  but  for  their 
remains,  which  have  been  entombed  in  the  earth,  what  has  been  the  cause  of  the  dis- 
appearance of  each  successive  race  ?  How  is  it  that,  at  a  given  epoch,  these  animated 
forms  have  on  a  sudden  stood  forth,  and  after  continuing  for  a  time,  as  suddenly  dis- 
appeared ?  Why  is  it  that  this  disappearance  does  not  take  effect  in  a  gradual  way, 
but  is  so  abrupt  an  affair  as  to  serve,  in  the  hands  of  geologists,  the  purpose  of  marking 
off  one  epoch  from  another  1  Do  not  these  total  disappearances  point  to  external  for- 
ces of  the  most  extensive  operation  as  the  agents  that  have  been  at  work,  and  the  end 
and  object  of  those  extinctions  the  production  of  physical  results  1 

21.  As  the  cause  of  extinction  of  those  innumerable  tribes  of  life  which  have  inhab- 
ited the  earth  is  directly  traceable  to  physical  events,  for  individuals,  also,  the  same  law 
holds  good.  On  the  American  Continent  the  mastodon  is  no  longer  seen,  though  but  a 
short  time  has  elapsed  since  all  the  rich  valleys  were  thronged  with  those  enormous  ele- 
phants, and  still,  in  the  salt-licks,  their  bones  are  disentombed  along  with  those  of  animals 
that  are  here  with  us.  Nor  is  it  alone  to  obscure  and  unintelligent  orders  of  life  that  this 
law  of  extinction  applies.  Within  the  periods  of  history,  have  not  the  same  things 
happened  ?  The  founders  of  the  greatest  empires  and  repul)lics  have  ceased  to  exist. 
Among  us,  what  has  become  of  that  ancient  people  who  built  the  extensive  structures 
in  the  Western  States  1  Their  name  and  every  recollection  of  them  have  passed  away. 
Even  before  our  eyes,  is  not  the  same  thing  happening  1  From  the  Atlantic  States, 
have  not  the  Indian  races  nearly  disappeared  ?  They  are  borne  by  the  tide  of  civili- 
zation across  the  great  Valley  of  the  Mississippi.  Among  them  one  tribe  after  another 
is  swept  away.  The  blood  of  the  Mandans  has  ceased  to  flow  in  the  veins  of  a  single 
liuman  being.  Worn  down  by  famine,  by  war,  and  by  pestilence,  these  children  of  the 
forest  recede  before  that  civilization,  the  benefits  of  which  they  obstinately  refuse  to 
receive,  and,  clinging  with  an  uncontrollable  instinct  to  a  wandering  and  savage 
life,  accompany  the  wild  animals  to  the  remoter  woods,  or,  driven  by  their  necessities, 
from  time  to  time  return  among  us,  and  beg  in  the  City  of  Washington  for  a  blanket 
and  a  little  bread. 

22.  Yet  among  them  there  are  men  capable  of  the  highest  emotions  and  the  most 
noble  deeds.  The  hand  of  Providence  presses  upon  the  Indian.  The  race,  like  each 
individual  of  it,  submits  in  silence  to  an  irreversible  doom.  From  the  day  when  organ- 
ization first  commenced  on  the  surface  of  the  earth,  the  law  which  it  has  followed  has 
been  a  law  of  progress  and  of  evolvement.  A  myriad  types  of  life  have  been  created, 
and  myriads  of  hving  forms  produced,  and  the  last  is  the  highest.    Even  with  us  the 


8 


INFLUENCE  OF  CLIMATE. 


same  thing  is  going  on  :  advances  in  knowledge  are  advances  in  power.  The  civihzed 
man  of  these  days  is  a  wholly  different  being  from  the  man  who  lived  a  thousand  years 
ago,  and  the  conditions  which  determine  his  position  have  totally  changed.  With  us 
the  position  both  of  empires  and  of  individuals  is  fixed  by  the  possession  of  knowledge — 
knowledge  which  is  incessantly  on  the  advance.  Wherever  intelligence  has  been  given, 
there  is  a  requirement  to  join  in  the  advancing  march.  The  Indian  stands  still,  and 
the  penalty  is  death. 

23.  These  severe  results  are  brought  about  by  universal  laws — laws  which  were 
not  intended  for  individual  cases.  In  the  system  of  the  universe  an  individual  is  not 
known,  but  action  takes  place  on  masses.  Nor  are  the  laws  of  Nature  ever  bent  to  give 
benefits  to  or  bring  punishment  on  any  individual.  They  go  into  effect  with  an  inex- 
orable decision.  The  earth  in  her  course  pursues  an  irresistible  march,  and  tides  rise 
and  fall  in  the  sea  with  a  fixed  fatality.  In  the  affairs  of  men  the  same  unwavering 
destiny  is  observed,  and  whether  it  be  in  the  case  of  an  empire  or  a  man,  resistance  to 
the  course  of  events  ends  in  an  inevitable  doom.  He  who  resists  the  progress  of  civi- 
lization, meets  the  same  fate  as  he  who  resists  the  cataract  of  Niagara.  There  is  no 
waywardness  in  Providence,  no  partialities,  and  no  hates.  It  is  written,  "  He  maketli 
his  sun  to  shine  on  the  good  and  the  evil,  and  sendetli  his  rains  on  the  just  and  on  the 
unjust." 

24.  From  these  considerations,  therefore,  we  may  gather  that  the  laws  of  Nature 
contain  provisions  for  the  extinction  and  removal  of  successive  races  ;  operations  which 
are  carried  on  by  the  action  of  physical  powers.  As  the  death  of  an  individual  arises 
from  the  action  of  external  agents,  so,  in  the  same  manner,  does  the  disappearance  of 
a  tribe :  and  hence  we  see  that,  as  existence  is  under  this  control,  it  cannot  take  place 
except  when  physical  circumstances  conspire ;  as  they  change,  so,  also,  must  the  vari- 
ous forms  of  life  undergo  corresponding  mutations. 

25.  In  the  constitution  of  all  organized  beings,  water  enters  as  the  leading  ingredient 
of  their  fluid  parts ;  it  is  therefore  obvious  that  there  is  a  very  limited  range  of  temper- 
ature in  which  the  processes  of  life  can  be  carried  on.  These  thermometric  limits  are 
between  32°  and  212°  Fah.    Life,  therefore,  is  comprised  within  a  range  of  180  degrees. 

26.  In  this  manner  we  might  proceed  to  show  how  the  existence  of  individuals  and 
races  is  completely  determined  by  external  conditions.  How,  for  the  same  reason  that 
an  individual  dies,  so  too  does  a  tribe  become  extinct.  Pursuing  these  considerations, 
we  might  show  how  closely  the  development  of  the  intellect  itself  is  connected  with 
them  ;  we  might  compare  the  effect  of  climates  in  the  torrid,  the  temperate,  and  the 
frigid  zone,  and  show  how  history  bears  out  the  truth  of  these  views.  We  might  ap- 
peal to  individual  experience  for  the  enervating  effects  of  hot  climates,  or  to  the  com- 
mon understanding  of  men  as  to  the  great  control  which  atmospheric  changes  exercise, 
not  only  on  our  intellectual  powers,  but  even  on  our  bodily  well-being.  It  is  within  a 
narrow  range  of  climate  that  great  men  have  been  born.  In  the  earth's  southern  hemi- 
sphere, as  yet,  not  one  has  appeared,  and  in  the  northern  they  come  only  within  certain 
parallels  of  latitude.  I  am  not  speaking  of  that  class  of  men  who,  in  all  ages  and  in 
every  country,  have  risen  to  an  ephemeral  elevation,  and  have  sunk  again  into  their  na- 


I 


INFLUENCE  OF  TElViPERATURE.  9 

live  insignificance,  so  soon  as  the  causes  which  have  forced  them  from  obscurity  cease, 
but  of  that  other  class,  of  whom  God  makes  but  one  in  a  century,  and  gives  him  a 
power  of  enchantment  over  his  fellows,  so  that  by  a  word,  or  even  by  a  look,  he  can 
"  electrify,  and  guide,  and  govern  mankind." 

27.  It  was  a  beautiful  idea  of  some  of  the  English  chemists  of  the  seventeenth  cen- 
tury, that  our  earth  is  nothing  more  than  an  incrusted  star — a  star,  the  light  of  which 
has  gone  out.  They  gathered  this  from  the  well-known  phenomena  of  hot  springs, 
and  the  increasing  temperature  of  deep  cavities.  In  their  opinion,  there  was  a  sun  in 
the  earth  beneath,  as  well  as  a  sun  in  the  firmament  above.  These  views  are  essen- 
tially the  same  as  those  which  are  now  received  by  geologists,  who  almost  universally 
admit  the  doctrine  of  a  central  heat,  the  surface  having  cooled  down  nearly  to  a  con- 
dition of  equilibrium.  Equally  beautiful  is  the  idea  of  M.  Poisson,  that  the  sun,  in  his 
movements  through  space,  successively  carries  his  attendant  planets  through  regions  of 
variable  temperatures — temperatures  which  are  variable  by  reason  of  the  different  amount 
of  stellar  radiation  which  crosses  them,  and  that,  for  many  ages  past,  he  has  been  com- 
ing from  a  warmer  to  a  colder  space.  Admitting  this  to  be  a  true  representation  of  the 
fact,  the  phenomena  which  we  witness  are  such  as  should  take  place.  If  a  great  mass 
of  rock  was  brought  from  the  equator  suddenly  into  the  polar  regions,  it  would,  from 
being  exposed  there  to  a  low  temperature,  commence  to  radiate  its  heat,  and  in  a  very 
short  time,  if  examined,  would  be  found  coldest  on  the  surface,  with  a  temperature  in- 
creasing towards  its  centre,  that  increase  being  at  first  rapid,  but  the  heat  subsequently 
becoming  uniform.  Such  a  rock  Poisson  regards  as  being  a  miniature  representation 
of  the  earth. 

28.  Which  ever  of  these  hypotheses  is  true,  of  one  thing  we  are  certain,  that  the  sur- 
face temperature  of  the  globe  has  undergone  periodic  changes,  and  with  these  chan- 
ges, not  only  has  the  distribution  of  plants  and  animals  varied,  but  general  disturbances 
have  taken  place  in  the  types  of  existing  species.  Those  creations  and  extinctions  to 
which  allusion  has  been  made  were  all,  undoubtedly,  connected  with  these  thermal  dis- 
turbances. There  can  be  little  doubt  that  the  mastodontoid  family  was  destroyed  by  a 
general  reduction  of  temperature.  With  modifications  of  the  distribution  of  this  all- 
pervading  agent,  changes  in  the  distribution  of  organized  beings  must  of  necessity  ensue. 
Even  with  us,  were  the  mean  temperature  to  rise  by  a  few  degrees,  the  great  mamma- 
lia of  the  torrid  zone  would  push  their  excursions  to  the  north  and  south.  The  Ben- 
gal tiger  would  leave  his  jungle,  and  press  himself  into  higher  latitudes.  It  would  re- 
quire but  a  slight  meteorological  change  to  bring  the  orange-tree  into  the  Middle  States, 
or  to  tempt  the  turkey-buzzard  far  to  the  north  of  New-York.  And,  on  the  other 
hand,  should  a  reverse  action  take  place,  and  the  mean  temperature  descend  by  a  few 
degrees,  the  indigenous  animals  and  plants  of  the  Northern  States  must  pass  to  a  lower 
latitude,  or  become  extinct.  The  distribution  of  organized  forms  is,  therefore,  directly, 
and  their  very  existence  indirectly,  determined  by  the  distribution  of  heat.  Who,  then, 
can  doubt  that  all  living  beings  depend  on  physical  force  1 

29.  If  these  things  are  true  with  respect  to  the  organized  forms  which  are  known 
to  have  existed  in  former  times — and  we  examine  the  relations  obtaining  between  the 

B 


IQ  RELATION  BETWEEN  ANIMATED  FORMS  AND  THE  ATMOSPHERE. 

present  climates  of  the  earth's  surface  and  the  animals  that  reside  on  it — how  abundant 
is  the  proof  that  all  are  still  under  the  control  of  physical  agents !  Things  being  so, 
and  each  species  having  its  assigned  spot,  from  which  it  does  not  wander,  for  many 
ages  the  same  inhabitauts  are  found  in  the  same  places.  They  change  as  individuals 
only :  the  old  decline  and  die  away,  the  young  spring  up  in  their  stead.  I  do  not 
know  whether,  in  taking  these  general  reviews  of  the  earth,  we  ought  to  have  any  re- 
gard for  individuals ;  whether  we  ought  to  consider  them  as  they  come  into  existence 
and  exhibit  the  strength  and  vigour  of  youth,  and  the  decrepitude  and  languor  of  old 
age.  Life  and  death  are  familiar  to  us,  because  as  individuals  we  have  a  deep  personal 
interest  in  them ;  but  the  cares  of  Nature  are  beyond  a  day.  In  the  Drama  of  the  Universe, 
each  actor  performs  his  part,  whether  leading  or  obscure,  and,  though  he  may  retire 
from  the  scenes,  the  play  goes  forward  to  its  catastrophe.  Whether  it  be  an  individual 
or  a  race,  each,  by  the  actions  of  its  life,  has  given  some  turn  to  the  general  course  of 
events.  In  the  undulations  that  circle  on  a  quiet  lake,  each  particle  alternately  rises 
up  and  sinks  into  repose;  but  that  particle,  minute  as  it  was,  that  motion,  small  as  it 
might  be,  was  al)solutely  necessary  to  keep  up  the  onward  motion  of  the  waves.  Under 
this  point  of  view,  the  destiny  of  each  individual  is  connected  with  the  destiny  of  the 
world. 

30.  With  each  breath  that  I  have  drawn  since  I  came  into  the  world,  I  have  nn- 
pressed  a  change  on  the  earth's  atmosphere.  One  year  with  another  I  have  removed 
therefrom  eiglit  hundred  pounds  of  oxygen  gas,  and  have  cast  into  it  a  much  greater 
weight  of  carbonic  acid,  made  up  of  the  detritus  of  atoms  which  have  been  dismissed 
from  the  system  as  unfit  for  the  continuance  of  life.  It  is  the  express  function  of  the 
act  of  respiration  to  produce  this  result.  That  carbon,  thus  given  forth,  was  received 
in  the  form  of  food,  it  sated  the  cravings  of  hunger,  or  gratified  those  animal  pleasures 
which  depend  on  the  sense  of  taste.  Taken  by  appropriate  apparatus  from  the  stom- 
ach, it  passed  into  the  circulation,  and,  grouped  with  other  materials,  it  changed  into 
blood ;  and  now,  by  capillary  attraction,  aided  by  the  action  of  the  heart,  it  was  car- 
ried to  every  part  of  the  body,  and  became  a  constituent  of  a  living  being — a  temporary 
constituent  only.  As  we  have  said,  it  is  a  vulgar  error  that  the  distinction  between  a 
living  and  a  dead  body  is  in  the  circumstance  that  the  former  can  maintain  itself  with- 
out change,  while  the  latter  undergoes  putrefactive  decomposition.  Both  equally  pu- 
trefy, or,  if  there  be  a  difference,  the  change  goes  on  most  quickly  in  a  living  system. 
The  true  distinction  rests  in  this,  that  a  living  body  is  accommodated  with  machinery 
to  remove  the  decomposing  atoms,  and  hence  the  process  is  conducted  in  a  regulated 
way.  A  chemist  looks  upon  life  as  made  up  of  unceasing  deaths.  And  those  atoms 
of  carbon  are  removed  because  their  life  is  over,  and  they  pass,  by  the  action  of  the 
lungs,  into  the  atmosphere  as  carbonic  acid  gas. 

31.  But  their  function  is  not  ended.  There,  it  is  true,  they  commingle  with  the 
elements  of  the  air,  and  are  no  longer  fit  to  support  the  respiration  of  man.  The  ways 
of  Nature  are  marvellous.  This  gas,  cast  away  by  animal  forms,  serves  now  as  food 
for  plants  and  trees.  Under  the  influence  of  the  solar  beams,  the  leaves  that  have  been 
unfolded  in  spring,  spreading  their  broad  expansions  to  the  air,  commence  the  decompo- 


4 


/ 

RELATION  BETWEEN  ANIMATED  FORMS  AND  THE  ATMOSPHERE. 


11 


sition  of  these  effete  atoms ;  they  undo  what  the  annual  system  has  done  ;  they  release 
the  oxygen  once  more,  and  restore  it  to  the  air  for  the  use  of  animals,  and  appropriate 
the  carbon  to  their  own  structures.  From  it  they  fashion  their  flowers,  from  it  they 
build  up  their  stems  or  perfect  their  fruit,  and  prepare  once  more  food  for  man. 

32.  Thus  do  these  carbonaceous  atoms  run  through  a  cycle  of  change,  passing  from 
animals  to  vegetables,  and  back  again  from  vegetable  to  animal  systems ;  and  the  Sun, 
to  whom  Nature  has  given  the  charge  of  these  wonderful  operations,  from  age  to  age 
furnishes  his  unfading  beams  (Ap.,  824).  He  sinks  below  the  horizon  at  night,  and  re- 
tires to  the  south  in  the  winter,  and  performs  his  work  by  regulated  and  measured 
steps.  Even  at  the  polar  regions,  where  there  is  no  morning,  no  noon,  no  evening,  no 
midnight,  where  there  is  no  rising  nor  setting  of  a  star,  the  great  orb  peers  above  the 
horizon,  "  and  under  the  influence  of  his  midnight  beams,  trees  and  plants  run  through 
the  same  series  of  changes  in  a  few  weeks,  which  they  accomplish  in  months  under 
the  purple  skies  of  beautiful  Italy." — Berzelius. 

33.  As,  thus,  we  see  the  sun  performing  the  office  of  a  great  life-giver,  and  know 
that  all  his  movements  are  accomplished  by  the  operation  of  mechanical  laws,  as  we 
understand  that  all  these  multiplied  motions  are  executed  by  the  action  of  one  attract- 
ive force,  it  would  seem  as  though  Nature,  on  the  great  scale,  called  on  us  to  recog- 
nise the  agency  of  mechanical  laws  in  regulating  the  processes  of  hfe. 

34.  The  carbonaceous  matter  which  has  flowed  through  the  heart  of  man  as  blood, 
is  transferred,  by  respiration,  to  the  air,  and  aids  in  the  formation  of  forest  trees  or 
painted  flowers.  The  Asiatics,  with  whom  have  originated  all  the  varieties  of  pagan 
creeds  that  have  spread  to  any  extent  in  the  world,  believed  in  a  transmigration  of 
souls ;  they  would  have  been  much  nearer  the  truth  had  they  believed  in  a  transmigra- 
tion of  bodies.  The  coal  that  we  burn  is  the  remains  of  forests  which,  in  former  ages, 
were  thronged  with  living  things — forests  that  sprang,  as  do  the  trees  with  us,  from 
gases  that  were  formed  from  the  respiration  of  animals — but  of  animals  that  are  all  extinct, 

35.  Atmospheric  air  is,  then,  the  grand  receptacle  from  which  all  hving  things  spring, 
and  to  which  they  all  return.  It  is  the  cradle  of  vegetable,  and  the  coffin  of  animal 
life.  Made  up,  as  it  is,  of  atoms  that  have  once  lived,  that  have  run  through  innumer- 
able cycles  of  change,  the  aspect  of  purity  it  presents  conceals  too  well  its  history.  In 
its  ethereal  expanse  are  crowds  of  atomic  forms  that  have  once  blossomed  as  flowers, 
or  participated  in  the  pleasures  and  pains  of  animal  life.  Their  former  function  dis- 
charged, these  atoms  that  are  dead  await  their  turn  of  reorganization  once  more. 
They  occupy  themselves  in  transmitting  the  many-coloured  beams  of  light,  or  moving 
in  vibration  to  the  tones  of  music.  A  condition  so  tranquil  suits  well  their  former  state 
and  future  destiny.  The  remains  of  wild  beasts,  or  of  more  ferocious  men,  disappear 
in  this  general  tomb  of  dying  atoms,  and  after  a  time  are  reorganized  by  the  solar 
beams  once  more,  perhaps  in  those  pensive  wild  flowers  that  blossom  unseen  in  the 
gloomy  wastes  of  an  American  forest. 

36.  We  know  that  the  daily  rotation  of  our  earth  on  her  axis  determines  periodic 
observances  in  the  functions  of  organized  beings,  and  Axes  their  times  of  activity  and 
sleep ;  a  similar  result  attends  upon  her  yearly  motion  in  her  orbit.  How  is  it  that,  in 
our  latitudes,  trees  and  plants  awake  at  the  coming  of  spring,  and  put  forth  their  leaves 


12 


INFLUENCE  OF  CURRENTS  IN  THE  AIR  AND  IN  THE  SEA. 


and  flowers,  and  then  sink  again  into  their  annual  slumber  1  How  is  it,  also,  that  wild 
birds  and  beasts  conform  in  their  habits  to  the  progress  of  the  seasons,  and  at  one  time 
prepare  to  bring  forth  their  young,  and  at  another  anticipate,  with  a  provident  foresight, 
the  coming  winter  ?  Those  great  migrations  of  fishes  that  take  place  at  given  seasons, 
and  which  are  even  connected  with  the  well-being  and  wealth  of  nations,  are  deter- 
mined by  the  occurrence  of  certain  epochs.  It  is  no  explanation  of  these  curious  facts 
to  say  that  they  depend  on  other  facts  like  themselves — that  an  animal  sleeps  by  night, 
because  his  prey  is  also  asleep  ;  that  a  fish  migrates  at  those  periods  when  his  instincts 
tell  him  that  the  food  on  which  he  lives  is  abundant.  If,  in  any  of  these  cases,  we 
pass  from  fact  to  fact,  we  uniformly  come,  at  last,  to  the  same  conclusion,  that  all  these 
incidents  are  directed  by  astronomical  events ;  that  the  sun  not  only  determines  peri- 
ods of  awakening  and  sleep,  of  growth  and  decay,  but  that  there  is  also  committed  to 
him  a  control  and  regulation  over  all  the  movements  of  animated  beings  on  the  face  of 
the  globe.  It  is  the  luminous  rays  of  that  distant  star  which,  falling  perpendicularly, 
produce  the  luxuriant  vegetation  of  tropical  regions,  and  his  rays  of  heat  which  debili- 
tate and  enervate  the  human  race.  It  is,  in  the  polar  regions,  the  obliquity  of  those 
beams  which  suffers  the  ground  to  be  always  covered  with  snow,  and  makes  those  in- 
hospitable countries  almost  without  inhabitants.  The  trade  winds,  also,  which  blow 
uninterruptedly  for  ages,  carry  away  towards  the  poles  immense  quantities  of  oxygen 
gas,  which  the  green  parts  of  plants  throw  into  the  atmosphere  of  the  torrid  zone. 
That  oxygen  is  evolved  by  light,  and  is  then  disseminated  by  heat.  In  the  sea  the 
same  influence  which  thus  presides  in  the  air  is  also  at  work.  The  Gulf  Stream, 
which  issues  from  the  Mexican  waters,  witli  its  temperature  elevated  by  solar  action, 
determines  the  distribution  of  the  Atlantic  fishes ;  the  northern  whale  avoids  its  oflen- 
sive  warmth,  and  on  its  sides  shoals  congregate  which  delight  in  a  more  genial  heat. 
As  it  approaches  the  coasts  of  Europe,  and  spreads  out  into  a  fan-like  form,  the  va- 
pours that  rise  from  it  give  forth  their  latent  heat  to  the  air,  and  moderate  the  climates 
of  England  and  France.  The  coldness  and  sterility  of  corresponding  latitudes  in 
America  is  there  replaced  by  a  better  temperature,  and  agriculture,  the  arts  of  life,  sci- 
ence and  literature,  have  there  reached  their  greatest  perfection.  This  physical  agent, 
thus  eternally  but  invisibly  continuing  its  operation,  produces  a  thousand  events  in 
which  its  agency  is  only  remotely  traced ;  nor  are  those  influences  limited  to  mere 
physical  results ;  they  stand  in  connexion  with  the  progress  of  society  and  the  evolu- 
tion of  mind.  A  full  development  of  the  reasoning  faculty  can  only  take  place  where 
physical  circumstances  conspire.  It  is  to  the  climate  of  England  and  France  that  the 
human  race  is  indebted  for  the  intellect  of  Newton  and  Laplace. 

37.  In  these  remote  events,  which  thus  originate  among  the  ordinary  phenomena  of 
the  natural  world,  and  strike  us  forcibly  when  we  trace  them,  step  by  step,  from  their 
origin  to  their  result,  we  are  prone,  at  a  casual  glance,  to  give  too  much  weight  to  in- 
tervening influences,  and  forget  the  final  cause.  Would  it  be  too  much  to  assert  that, 
with  returning  seasons,  periods  of  vegetation,  and  the  distribution  of  animated  nature, 
astronomical  occurrences  likewise  direct  a  thousand  of  those  daily  movements  which 
are  taking  place  in  every  part  of  the  world  1  There  is  no  harvest  which  is  gathered, 
no  famine  which  desolates,  that  has  not  sprung  from  an  immediate  connexion  with 


INDEPENDENCE  OF  THE  HIGHER  ANIMALS  OF  EXTERNAL  AGENTS.  ^3 

them.  In  judfj;ing  from  a  narrow  circle  of  observation,  or  from  an  imperfect  experience, 
men  are  led  to  regard  these  as  fortuitous  affairs.  Are  they  not  rather  brought  about 
by  unfaiUng  and  unchangeable  causes  1  From  century  to  century,  the  sun  pours  forth 
his  undiminished  stores  of  light  and  heat ;  the  former,  out  of  inorganic  material,  con- 
structs molecules  that  are  organized,  and  from  them  builds  up  the  myriads  of  vegetable 
forms  which  are  destined  for  supporting  animal  life ;  the  latter,  controlling  the  move- 
ments of  inorganic  matter,  divides  into  climates  the  earth's  surface,  volatilizes  water 
from  the  sea,  sets  the  wind  in  motion,  and  directs  the  form,  duration,  and  movement  of 
the  clouds.  Tlie  primitive  force  which  is  at  work,  producing  these  vital  and  mete- 
orological phenomena,  undergoes  no  change  in  intensity  from  year  to  year;  it  is  there- 
fore expended  in  producing  the  same  amount  of  effect.  It  is  through  this  that  the 
droughts  of  one  country  are  contemporaneous  with  the  abundant  showers  of  another 
— the  famine  which  threatens  one  place  is  compensated  by  harvests  in  another.  As 
natural  laws  were  never  meant  for  operation  on  individuals,  but  for  action  on  systems 
and  masses,  incidental  vicissitudes,  such  as  those  of  which  we  are  speaking,  should 
never  guide  us  in  our  judgment  of  final  results.  Operating  with  an  unerring  certainty, 
and  with  an  unchangeable  force,  the  sun  carries  on  his  plastic  works,  as  the  earth,  in 
her  daily  rotations,  submits  herself  to  his  beams.  From  this  it  comes  to  pass,  that 
though  there  maybe  variations  in  the  lot  of  particular  nations  or  particular  individuals, 
the  common  interests  of  all  are  protected,  the  common  rights  of  all  are  upheld.  From 
the  very  beginning  of  things,  every  class  of  variation  has  been  determined — where  par- 
ticular climates  shall  fall,  where  particular  temperatures  shall  be  observed,  what  shall 
be  the  speed  of  vegetable  growth,  what  tribes  of  animals  shall  be  given — and  the  thing 
remains  fixed  and  unalterable. 

38.  One  of  the  most  striking  results  of  organic  chemistry  is  the  relationship  which 
it  discovers  between  animals  and  plants ;  the  former  constituting  an  apparatus  for  ox- 
ydation,  the  latter  an  apparatus  for  deoxydation.  Compared  together,  a  relation  of 
antagonism  exists  between  them.  Plants,  from  inorganic  matter,  construct  their  vari- 
ous tissues  and  parts  ;  these  are  consumed  by  animals,  and  forced  back  into  the  inor- 
ganic state.  It  is  therefore  plain  that  the  sun  is  the  great  formative  agent,  and  ani- 
mals are  the  destroyers.  If  we  consider  the  successive  races  of  organized  beings,  be- 
ginning from  the  lowest  and  passing  to  the  higher  tribes,  it  would  seem  as  if  the  gener- 
al idea  under  which  Nature  has  been  acting  is,  that  as  the  more  complex  structures  were 
evolved  to  emancipate  them  from  the  direct  control  of  external  physical  forces.  The 
vegetable  kingdom,  unendued  with  locomotive  powers,  deriving  its  existence  directly 
from  external  agents,  is  completely  under  their  control.  If  the  summer  is  too  brilliant, 
or  rains  do  not  fall,  a  plant  withers  and  dies.  In  the  same  manner,  the  lower  races  of 
animals  have  their  existence  determined  by  the  action  of  physical  causes  ;  if  these  be 
favourable,  they  flourish ;  if  unfavourable,  they  must  submit  to  an  inevitable  lot.  To 
tribes  that  are  higher,  to  a  certain  extent,  the  rigour  of  these  laws  is  remitted,  and  a 
certain  amount  of  independence  secured ;  the  African  lion  can  retire  to  a  shade  in  the 
middle  of  the  day  ;  yet  still  he  is  held  in  a  state  of  subjection,  and  instinctively  submits 
to  the  operation  of  an  overruling  power,  and  is  kept  to  the  sands  of  his  desert  from 
cool  and  temperate  climates.    The  sunbeam  is  his  chain.    In  man  alone  the  emanci- 


14 


INDEPENDENCE  OF  THE  HIGHER  ANIMALS  OF  EXTERNAL  AGENTS. 


pation  is  complete  ;  for  into  his  hand  Nature  has  committed  a  control  of  the  imponder- 
able principles.  It  matters  not  whether  it  be  in  the  torrid  zone  or  in  the  frigid,  he  tem- 
pers the  seasons  by  his  intellectual  power ;  he  resorts  to  every  artifice  of  clothing,  or 
to  the  warmth  of  fire ;  he  dissipates  the  natural  darkness  by  artificial  light.  Devel- 
oped by  civilization,  he  is  no  longer  a  prey  to  natural  accidents ;  if  the  harvests  of  his 
own  country  have  failed  him,  his  hands  have  created  commerce,  which  brings  him  an 
abundance  from  distant  places.  Unlike  even  those  races  which  are  next  below  him,  and 
which  instinctively  aim  at  the  result  he  so  perfectly  accomplishes,  he  does  not  wait 
upon  the  gifts  of  Nature,  but  compels  her  to  minister  to  him.  When  they  are  oppress- 
ed by  hunger,  whole  tribes  of  fishes  migrate  in  the  sea,  and  innumerable  flocks  of  birds 
direct  their  flight  to  distant  countries ;  hut  civilized  man,  without  calling  into  action 
his  own  locomotive  powers,  puts  his  arm  across  the  globe,  and  satisfies  his  wants. 

39.  What,  then,  are  the  final  impressions  left  upon  our  minds  by  these  general  con- 
siderations ?  They  teach  us  that  life  never  occurs  except  in  regions  to  which  the  im- 
ponderable agents  can  have  access,  an  observation  which  is  equally  true  of  vegetable 
and  of  animal  forms ;  that  elementary  organization  directly  or  indirectly  arises  from 
the  plastic  agency  of  those  all-pervading  forces.  Whether  we  consider  the  organic  or 
inorganic  world,  all  things  around  us  are  in  incessant  changes — changes  which  i-esult 
from  the  fixed  operation  of  invariable  laws ;  that,  of  the  successive  tribes  of  beings 
which  have  peopled  our  earth,  each  series  may  be  regarded  as  expressing  the  general 
relation  of  all  physical  agents  at  the  time  of  its  existence,  the  brilliancy  of  the  sun,  the 
pressure  of  the  air,  and  other  such  conditions ;  for  we  see  that,  between  those  condi- 
tions and  the  organization  of  the  structures  considered,  there  are  fixed  relations ;  that 
in  the  more  highly  comphcated  forms  of  beings,  mutations  more  readily  take  place,  and 
in  all,  time  enters  as  an  element ;  that  in  the  same  way  that  whole  races  have  disap- 
peared from  the  face  of  the  earth,  and  have  become  extinct,  so,  also,  do  individuals  die 
and  atoms  change  ;  that,  whatever  motion  is  accomplished,  or  whatever  change  is  brought 
about,  there  is  a  consumption  of  material  or  an  expenditure  of  force ;  that,  as  the  sur- 
face of  the  earth  is  continually  remodelled  by  physical  agents,  so  are  the  vicissitudes 
through  which  organized  forms  pass  determined  by  physical  powers,  and  bring  about 
physical  ends.  The  passage  of  a  comet,  never  more  to  return,  in  a  hyperbolic  orbit  past 
the  sun,  is  a  result  of  the  same  general  law  that  keeps  a  planet  revolving  in  repeated 
circles — the  extinctions  of  races  which  have  heretofore  taken  place,  or  which  are  going 
on  before  us,  are  not  brought  about  by  a  direct  intervention  of  supernumerary  forces, 
but  are  the  constant  result  of  those  which  are  always  in  action.  If,  moreover,  our 
thoughts  are  directed  to  the  relations  which  exist  between  climates  and  the  character 
of  races,  the  distribution  of  vegetables  and  animals ;  if  we  observe  the  antagonization  of 
these  great  classes  in  the  result  of  their  vital  processes,  their  position  as  respects  the  at- 
mosphere, the  control  which  astronomical  events  possess  over  everything,  the  action 
which  currents  in  the  air  or  currents  in  the  sea  exercise  over  the  distribution  of  anima- 
ted forms,  and  even  over  the  well-being  of  man,  we  surely  shall  have  but  little  difficulty 
in  understanding  that,  as  in  the  inorganic  world,  so  also  in  the  world  of  organization, 
those  all-pervading  forces  which  natural  philosophers  and  chemists  recognise  are  con- 
stantly employed. 


A  TREATISE 

ON  THE  FORCES  WHICH  PRODUCE 

THE  ORGANIZATION   OF  PLANTS. 


CHAPTER  I. 

ON   THE   ACTION   OF   THF,   SUNBEAMS   IN   PRODUCING   ORGANIZED  BODIES. 

Contents  :  Tlie  Growth  of  ConfervcB  in  Water. — Production  of  Green  Matter  hy  Spun 
Glass  and  Inorganic  Bodies. — It  requires  a  Sporule,  Cell,  or  Objective  Germ. — Mode 
of  Diffusion  of  Gases  into  Water. — Influence  of  Temperature  on  the  Process. — Action 
of  the  Suns  Rays  on  these  Gases. —  Tivo  Atmospheres  around  the  Earth. — Sources 
of  Supply  of  the  expended  Gases. 

Application  of  these  Principles  to  the  Production  of  Chlorophyl  in  Leaves. —  The  Di- 
gestion of  Plants. 

40.  If  we  expose  some  spring  water  to  the  sunshine,  though  it  may  have  been  clear 
and  transparent  at  first,  it  presently  begins  to  assume  a  greenish  tint,  and,  after  a  while, 
flocks  of"  green  matter  collect  on  the  sides  of  the  vessel  in  which  it  is  contained.  On 
these  flocks,  whenever  the  sun  is  shining,  bubbles  of  gas  may  be  seen,  which,  if  collected, 
prove  to  be  a  mixture  of  oxygen  and  nitrogen,  the  proportion  of  the  two  being  variable. 
Meantime  the  green  matter  rapidly  grows,  its  new  parts,  as  they  are  developed,  being 
all  day  long  covered  with  air-bells,  which  disappear  as  soon  as  the  sun  is  set.  If  these 
observations  be  made  on  a  stream  of  water,  the  current  of  which  runs  slowly,  it  will  be 
discovered  that  the  green  matter  serves  as  food  for  thousands  of  aquatic  insects,  which 
make  their  habitations  in  it.  These  insects  are  endued  with  powers  of  rapid  locomo- 
tion, and  possess  a  highly-organized  structure ;  in  their  turn,  they  fall  a  prey  to  the 
fishes  which  frequent  such  streams.  Organic  chemistry  teaches  that  it  is  the  office  of 
vegetable  life  to  form  from  inorganic  matter  organized  molecules,  and  furnish  them  as 
food  for  the  support  of  animals,  which  simply  assimilate,  but  do  not  fabricate  ;  we  must, 
therefore,  infer  that  the  fibrine,  the  albumen,  the  gelatine,  the  fat,  and  whatever  else  of 
those  compound  organic  molecules  is  required  for  the  support  of  fishes  and  insects,  are 
originally  formed  by  the  action  of  light  on  that  green  matter.  But  the  production  of  this 
substance  is  the  result  of  a  multitude  of  coincident  actions.  The  sunlight  is  the  agent 
which  directs  its  growth,  but  it  does  not  so  plainly  appear  what  is  the  body  from  which 
it  originally  springs,  and  on  which  the  light  exerts  its  influence ;  whether  it  comes  from 
microscopic  germs  which,  floating  about  in  the  air,  find  their  way  into  every  water,  or 


THE  GROWTH  OF  CONFEKV.^!;  IN  VVATEK. 


from  organic  cells  furnished  from  obscure  sources.  The  process  once  begun,  goes  on 
with  uniformity  and  rapidity  so  long  as  external  circumstances  are  favourable.  Some 
of  the  chemists  of  the  last  century  asserted  that  no  organic  nucleus  was  required,  and 
that,  on  putting  such  bodies  as  spun  glass  or  amianthus  into  a  vessel  of  water,  on  ex- 
posure to  light,  those  fibres  would  become  covered  with  bubbles,  and  the  water  would 
begin  to  turn  green.  But  I  have  found  that  these  are  experiments  which  do  not  suc- 
ceed except  under  circumstances  where  there  is  every  reason  to  suspect  the  incidental 
introduction  of  organic  matter.  Count  Rumford  has  stated  that  yellow  silk,  in  an 
unspun  state,  as  it  comes  from  tlie  worm,  will  cause  a  very  rapid  escape  of  oxygen  un- 
der the  influence  of  light.  It  is  probable,  however,  that  here,  as  in  the  former  cases, 
some  specific  organic  matter  has  been  introduced,  for  the  result  very  often  cannot  be 
obtained. 

41.  Similar  green  flocks  to  those  of  which  we  here  speak  are  also  found  on  the  sur- 
faces of  rocks  exposed  to  the  sea,  damp  walls,  and  other  places  where  there  is  a  con- 
stant moisture.  They  belong  to  the  algte  or  sea-weed  tribe  of  botanists.  The  con- 
fervse  are  thus  described  by  Professor  Lindley  :  "  They  consist  of  filaments,  generally 
simple,  and  are  formed  of  two  tubes,  of  which  one,  which  is  exterior  and  transparent, 
offers  no  trace  of  organization  to  the  most  powerful  eye,  so  that  it  might  be  called  a 
tube  of  glass,  contains  an  inner  articulated  filament,  filled  with  colouring  water  often 
almost  imperceptible,  but  at  other  times  a  very  intense  green,  purple,  or  yellow  colour." 

42.  Admitting  the  existence  of  some  objective  germ,  or  sporule,  or  point  on  which  the 
light  can  act,  we  are  able  to  give  a  pretty  clear  account  of  the  process  of  evolution  of 
the  green  matter.  When  pure  water  is  freely  exposed  to  atmospheric  air,  in  conse- 
quence of  the  quality  exhibited  by  all  substances  (Ap.,  75)  of  diffiising  into  the  inter- 
stices of  each  other,  there  begin  to  pass  into  it  the  different  gaseous  bodies  which  have 
access  to  it.  These  do  not  all  pass  with  the  same  facility,  nor  are  they  taken  up  to 
the  same  amount.  The  relative  quantity  of  each  follows  the  order  of  its  solubility.  Of 
the  three  gases  to  which  our  attention  must  be  directed  in  the  phenomenon  we  are  now 
considering,  oxygen,  nitrogen,  and  carbonic  acid,  carbonic  acid  passes  into  the  water 
with  the  greatest  speed,  oxygen  next,  and  nitrogen  much  more  slowly  (Ap.,  54-63). 
When  a  state  of  equilibrium,  therefore,  has  been  attained,  it  will  be  discovered  that  the 
pure  water  has  become  contaminated  by  the  presence  of  these  different  gases,  and  that 
they  exist  in  it  to  an  amount  represented  by  their  rate  of  solubility. 

43.  Spring  water  and  river  water,  therefore,  naturally  contain  these  different  gaseous 
substances,  which  are  intimately  connected  with  the  production  of  green  matter.  In- 
deed, from  the  very  mode  in  which  rivers  and  springs  are  fed,  the  solution  of  gaseous 
matter  in  them  is  completely  secured.  By  the  action  of  solar  heat,  vapours  are  rais- 
ed from  the  sea,  and  ascending  to  the  more  elevated  and  cooler  regions,  give  out  their 
latent  heat,  and  condense  into  vesicles  or  microscopic  drops,  the  aggregate  of  which 
forms  a  mist  or  cloud.  In  this  state  of  excessive  subdivision,  the  drops  are  brought  into 
a  perfect  contact  with  the  air,  and  an  absorption  of  its  constituents  takes  place.  To 
give  still  greater  security,  the  drops  of  rain,  which  are  nothing  more  than  those  vesicles 
coalesced,  as  they  come  down  from  the  sky  pass  through  the  subjacent  aerial  strata,  and 


AMOUNT  OF  WATER-GAS  DETERMINED  BY  HEAT.  7 

withdraw  as  much  gas  as  they  can  hold.  From  rains  which  thus  take  place  and  fall  on 
elevated  regions,  springs  and  rivers  arise,  containing,  therefore,  those  gaseous  materials 
which  have  been  obtained  from  the  atmosphere. 

44.  This  water-gas,  as  it  might  be  termed,  may  be  expelled  entirely  from  water  by 
boiling,  and  by  resorting  to  that  process  a  knowledge  of  its  constitution  has  been  obtained 
(Ap.,  93).  The  power  by  which  liquids  hold  gaseous  substances  in  solution  is  diminished 
by  heat,  and  as  soon  as  a  liquid  boils  all  extraneous  gas  is  carried  away,  because,  under 
those  circumstances,  the  vapour  which  is  generated  by  the  heat,  passing  in  bubbles 
through  the  mass  of  the  liquid,  exposes  itself  to  the  contained  extraneous  gas,  wiiich 
diffuses  rapidly  into  it,  and  escapes  away  when  the  bubbles  burst  on  the  top.  The  law 
of  equilibrium  of  diffusing  gases  teaches  us  that  this  process  can  only  end  in  the  total 
removal  of  the  contained  air  (Ap.,  47,  48).  Temperature  thus  controlling  the  quantity 
of  gas  dissolved  in  water,  we  can  easily  see  that  in  different  countries  the  relative 
amount  of  water-gas  will  differ,  and  that  its  mean  quantity  will  be  connected  with  the 
mean  annual  temperature,  and  in  those  seasons  during  which  vegetable  action  is  ad- 
vancing most  rapidly,  that  is,  in  spring  and  summer,  its  amount  will  be  determined  by 
the  mean  temperature  of  the  spring  and  summer  months.  In  tropical  climates  there 
should  be  less  of  this  gas  than  in  temperate,  but  in  the  use  which  is  to  be  made  of  it 
for  physiological  purposes  a  compensating  agency  appears ;  the  poorer  water  of  equa- 
torial countries  is  acted  on  by  a  more  brilliant  ray.  The  light  and  heat  of  the  sun 
here  stand  in  the  attitude  of  antagonizing  forces.  The  calorific  beams,  by  reason  of  their 
obliquity  in  the  more  polar  countries,  allow  an  increased  quantity  of  gas  to  be  dissolved 
in  water ;  the  luminous  beams  which  come  with  the  same  obhquity  have  therefore  more 
to  operate  on. 

45.  If  saline  matter  be  present,  it  acts  as  a  disturbing  agent ;  for  a  liquid  which  is 
impregnated  with  salt  has  its  solvent  powers  greatly  diminished.  Bulk  for  bulk,  there- 
fore, sea  water  must  contain  a  less  volume  of  gas  than  fresh  water. 

46.  The  gas  thus  included  in  the  interstices  of  water  discharges  many  important 
functions.  It  is  connected  with  the  physiological  operations  both  of  the  aninjal  and 
vegetable  world.  Being  twice  as  rich  in  oxygen  as  atmospheric  air  (Ap.,  93),  it  is  des- 
tined to  carry  on  the  respiratory  processes  of  those  classes  of  animals  whose  form  re- 
quires that  their  breathing  apparatus  should  be  of  the  most  compendious  figure,  and 
which  have  to  live  in  the  water.  Thus,  in  those  fishes  which  breathe  by  branchige,  the 
processes  of  combustion  which  are  to  go  on  in  their  systems  depend  on  a  supply  of  the 
concentrated  oxygen  of  water-gas.  The  fish  swallows  a  mouthful  of  water,  and  then, 
by  muscular  contraction,  drives  it  out  past  the  gills,  among  the  threadhke  fibrillae  in 
which  venous  blood  is  passing.  Inter-transfusion  in  an  instant  takes  place,  oxygen 
from  the  water-gas  flashes  into  the  venous  blood  and  arterializes  it,  carbonic  acid  si- 
multaneously comes  out,  and,  as  the  fish  moves  forward,  is  carried  away  in  the  current 
of  water, 

47.  A  mass  of  water  thus  containing  carbonic  acid,  oxygen,  and  nitrogen,  with  some 
germ,  or  sporule,  or  objective  point,  on  which  the  light  can  act,  is  exposed  to  the  sun. 
As  has  been  said,  a  bubble  of  gas  soon  makes  its  appearance,  and  growth,  with  a  devel- 

C 


18. 


TWO  DPXOMPOSITIONS  EFFECTED. 


opmeut  of  the  green  colour,  takes  place.  If  we  examine  the  changes  which  are  now- 
occurring  in  the  water,  we  find  that  the  carbonic  acid  is  disappearing,  and  oxygen  and' 
nitrogen  are  evolving.  The  growing  mass  increases  in  volume  and  weight.  After  a 
time,  if  proper  measures  are  taken  to  cut  off  any  farther  supply  of  carbonic  acid,  the  pro- 
cess conies  to  an  end.  But  as  conducted  naturally,  there  is  a  free  exposure  to  the  at- 
mosphere, from  which  the  acid  diffuses,  and  hence,  as  fast  as  its  removal  by  decompo- 
sition takes  place,  new  quantities  are  restored  again. 

48.  The  direct  action  which  is  accomplished  under  the  influence  of  the  light  is  the  total 
decomposition  of  the  carbonic  acid  gas.  Its  oxygen  is  evolved,  and  its  carbon  goes  to 
form  the  green  matter.  At  first  sight,  the  supposition  of  the  generality  of  chemical 
writers  seems  to  be  probable,  that  the  carbon  thus  evolved,  while  it  is  yet  in  a  nascent 
state,  unites  directly  with  the  elements  of  water,  and  produces  one  of  the  starch  family 
of  bodies.  But  the  constitution  of  the  green  matter  is  far  from  being  so  simple. 
Chemical  examination,  and  considerations  connected  with  the  nutrition  of  the  insects 
which  feed  on  it,  serve  to  show  that  it  contains  more  hydrogen  than  is  necessary  to 
convert  its  oxygen  into  water.    There  is  also  nitrogen  in  it. 

49.  In  the  air-bubbles  which  form  and  finally  escape,  we  find  not  pure  oxygen,  as  is 
generally  supposed,  but  a  mixture  of  oxygen  and  nitrogen  gases.  The  source  of  the 
former  is  unquestionably  to  be  referred  to  the  decomposed  carbonic  acid.  From  the 
circumstance  that  in  the  growing  mass  there  is  an  excess  of  hydrogen,  and  in  the  esca- 
ping air  a  large  quantity  of  azote,  it  would  seem  that  a  compound  having  the  same  ele- 
ments as  ammonia  has  been  acted  upon.  Nevertheless,  the  quantity  of  ammonia  which 
is  commonly  dissolved  in  spring  or  river  water  is  far  too  small  to  furnish  the  supply 
which  would  be  necessary.  It  seems,  therefore,  not  improbable  that  the  nitrogen  nat- 
urally dissolved  along  with  the  carbonic  acid  and  oxygen  in  the  water  is  indirectly 
connected  with  the  decomposition. 

50.  The  presence  of  carbon,  and  an  excess  of  hydrogen  in  the  green  matter,  proves 
that  the  sunshine  has  effected  two  different  decompositions,  the  decomposition  of  car- 
bonic acid  and  that  of  water. 

51.  At  this  stage  of  the  description,  let  us  pause  and  review  the  facts  which  have 
presented  themselves  to  us.  We  have  arrived  at  the  conclusion  that  all  the  solid  ma- 
terial under  consideration  is  produced  from  gaseous  matter  contained  in  water  in  a  state 
of  solution.  This  gaseous  matter  comes  directly  from  the  atmosphere;  it  contains  the  same 
constituents  as  the  atmosphere,  but  differs  from  it  in  having  them  in  a  different  proportion. 
We  see,  therefore,  that  there  are,  as  it  were,  two  atmospheres  enveloping  the  earth's  sur- 
face; one  of  these  is  the  air  which  we  breathe,  the  other  that  which  exists  dissolved  in  the 
sea,  and  in  those  streams  and  waters  that  are  found  on  the  surface  of  the  ground.  Between 
these  a  remarkable  relation  exists  :  in  the  common  atmosphere  the  proportion  of  oxygen 
to  nitrogen  is  nearly  as  one  to  four,  in  the  water-atmosphere  it  is  nearly  as  one  to  two. 

52.  Between  these  two  atmospheres,  a  constant  connexion  is  established,  through 
the  chemical  relations  of  water,  which  receives  its  gaseous  contents  from  several  differ- 
ent sources.  Part  of  its  carbonic  acid  it  obtains  by  direct  absorption  from  the  superin- 
cumbent air,  and  part  is  given  to  it  by  the  respiration  of  fishes  and  other  aquatic  ani- 


MODE  OF  DIFFUSION  OF  GASES  INTO  WATER.  29 

inals ;  for  animal  life,  though  carried  on  beneath  the  surface  of  this  liquid  medium,  has 
the  same  chemical  object  in  view  as  when  carried  forward  on  the  surface  of  tlie  earth; 
it  ends  in  the  conversion  of  carbon  into  carbonic  acid,  of  hydrogen  into  water,  and  an 
evolution  of  heat.  Its  oxygen,  in  the  same  manner,  has  two  sources  of  supj)lj,  direct 
absorption  from  the  air,  as  in  the  former  instance,  and  a  still  more  concentrated  store 
in  those  little  air-bells  that  cover  the  green  vegetable  matter  as  long  as  the  sun  is  shi- 
ning; in  these  the  volume  of  oxygen  is,  on  an  average,  double  that  of  the  nitrogen. 
Upon  them  the  water  exerts  its  solvent  powers,  removes  as  nuu;h  as  it  can  carry  away, 
and  the  bubble  then  floats  to  the  top  of  the  water,  and  escapes  out  into  the  air.  From 
the  same  two  sources  nitrogen  is  also  procured.  With  these  abundant  supplies,  there- 
fore, furnished  partly  from  the  vital  changes  which  are  taking  place  in  its  mass,  and 
partly  from  the  external  air,  the  constitution  of  the  water-atmosphere  is  kept  up  unim- 
paired. 

53.  All  the  carbon  which  we  thus  find  in  the  green  matter  comes  from  dissolved  car- 
bonic acid,  all  the  hydrogen  from  water  or  ammonia ;  the  latter  also  furnishes  a  cer- 
tain quantity  of  nitrogen,  other  portions  of  which  are  obtained,  without  any  decompo- 
sition, from  that  which  has  been  dissolved  out  of  the  atmosphere.  The  general  prin- 
ciples of  life  carried  on  in  the  water  are  modelled  on  the  same  idea  as  in  the  case  of 
life  carried  on  in  the  air.  In  both  cases,  vegetables  act  as  the  great  formative  agents, 
and  animals  as  the  destructive  power;  and  in  both,  the  source  and  origin  of  action  is 
to  be  found  in  the  beams  of  the  sun.  For  nearly  two  centuries,  physical  science  has 
fully  admitted  the  agency  of  that  central  star  as  the  great  seat  of  mechanical  force, 
which  retains  the  different  planets  in  their  orbits.  It  is  only  of  late  years  that  we  are 
beginning  to  recognise  his  agency  as  the  author  of  organization  and  life,  who  lays  up, 
with  an  almost  provident  foresight,  in  vegetable  productions,  stores  oi  light  and  heat 
for  the  use  of  the  animal  world.  The  coal-fields  which  furnish  us  with  fuel  are  the  re- 
mains of  primeval  forests,  among  the  branches  of  which  birds  nestled  at  night ;  and  the 
warmth  that  we  receive  from  them,  and  the  light  that  they  give  us,  have  been  safely 
stored  up  for  us  for  thousands  of  centuries.  Those  little  insects,  also,  which  at  certain 
seasons  cause  the  sea  to  shine  with  a  phosphorescent  light,  derive  their  glow  remotely 
from  the  vegetable  kingdom ;  and  the  fireflies,  which,  in  such  countless  multitudes,  on 
a  summer  evening  in  Virginia,  make  the  grass  and  trees  glitter  with  their  intermitting 
beams,  are  only  pouring  forth  again  rays  which  once  came  from  the  sun. 

54.  I  have  thus  far  considered  the  process  of  evolving  green  matter  from  a  primitive 
cell  or  seed  placed  under  water.  It  is  now  proper  to  generalize  on  these  views,  and  to 
show  that,  so  far  from  this  being  an  insignificant  case,  it  represents  fully  all  that  goes  on 
in  the  vegetable  tribes,  whether  they  live  under  the  water  or  in  the  air.  Let  us,  therefore, 
proceed  at  once  to  investigate  what  takes  place  in  the  case  of  plants  which  are  of  a 
more  complex  character,  and  higher  in  the  scale  of  creation.  If  a  few  garden  seeds  of 
any  kind  are  sown  in  a  flower-pot,  and  caused  to  germinate  in  a  dark  room,  after  a 
while  it  will  be  perceived  that  they  can  grow  for  a  certain  space  in  the  absence  of  light; 
their  young  leaves,  if  any  should  be  put  forth,  are  of  a  yellow  or  gray-white  colour,  and 
they  soon  fade  away  and  die.    But  if  these  plants  be  brought  out  into  the  light,  they 


20 


PRODUCTION  OF  CHLOROPHYL  IN  LEAVES. 


presently  begin  to  turn  green,  they  unfold  their  leaves,  and  evolve  their  different  parts 
in  a  natural  way.  From  day  to  day  their  weight  increases,  and  chemical  analysis  shows 
that  they  are  fixing  carbon,  hydrogen,  oxygen,  and  azote.  If  they  be  made  to  grow  in 
confined  glass  vessels,  under  such  circumstances  that  an  examination  can  be  instituted 
on  the  changes  they  are  impressing  on  the  atmosphere,  it  is  discovered  that  they  are  con- 
stantly abstracting  carbonic  acid  from  it,  and  as  long  as  the  sun  shines  on  them,  or  as 
long  as  they  are  exposed  to  bright  daylight,  they  continue  appropriating  carbon  and  ex- 
haling a  mixture  of  oxygen  and  nitrogen.  The  continuance  of  their  growth  depends 
on  a  continued  supply  of  the  acid  gas  in  due  quantities.  The  leading  facts  which  are 
here  mentioned  were  discovered  during  the  last  century  by  Priestley,  who  found  that 
when  leaves  of  any  kind  are  placed  in  water,  which  holds  carbonic  acid  gas  in  solution, 
they  evolve  oxygen  when  in  the  sunshine.  It  is  not  pure  oxygen,  but  a  mixture  of  that 
gas  with  azote  (Ap.,  794).  It  has  been  objected  that  no  conclusion  can  be  drawn  from 
experiments  conducted  in  this  way  in  regard  to  vegetable  functions,  for  a  plant  which 
is  fitted  to  carry  on  its  living  processes  in  the  atmosphere  is  placed  in  an  unnatural  con- 
dition when  immersed  in  a  vessel  of  water.  But  there  is  much  less  force  in  this  objec- 
tion ihan  might  at  first  sight  appear.  A  leaf,  exposed  to  the  air,  does  not  absorb  and  act 
upon  carbonic  acid  gas  as  a  gas ;  its  tissues  and  parts  are  saturated  with  water,  which  has 
been  thrown  up  by  the  capillary  force  of  the  spongioles,  or,  on  some  occasions,  obtained 
directly  from  ihe  atmosphere  by  the  beautiful  process  of  nightly  radiation  to  the  sky,  and 
corresponding  precipitation  of  drops  of  dew  ;  this  water,  thus  penetrating  every  part  of 
the  succulent  structure,  is  the  medium  through  which  carbonic  acid  is  absorbed  and  de- 
composed. In  point  of  fact,  therefore,  though  plants  may  live  in  air,  their  mode  of  dis- 
charging this  function  is  the  same  as  though  they  were  living  in  water,  for  the  absorp- 
tive force  of  that  liquid  is  called  into  play,  and  carbonic  acid  is  presented  in  a  dissolved 
state,  and  the  case,  in  reality,  becomes  nothing  more  than  a  repetition  of  what  goes  on 
when  water-plants  are  digesting.  In  very  many  instances  Nature  takes  extraordinary 
pains  to  secure  a  rapid  supply  of  the  acid.  In  water-plants,  which  are  often  fixed,  re- 
liance is  had  on  currents  which  are  established  by  variations  of  specific  gravity,  so  as 
fast  as  one  portion  of  water  has  approached  to  the  place  of  decomposition  and  surren- 
dered its  gas,  it  is  pressed  away  by  those  around,  which  are  about  to  discharge  the  same 
duty.  But  in  aerial  plants,  the  digestive  organ  unfolds  a  broad  surface  to  the  atmo- 
sphere, and  the  calorific  beams  of  the  sun  descending  on  it,  by  reason  of  its  dark  col- 
our are  readily  absorbed ;  a  warm  current,  which  is  easily  established  in  a  mobile  fluid 
like  air,  rises  rapidly ;  and,  as  if  this  were  not  enough  in  multitudes  of  instances,  the 
trembhng  leaves  are  set  upon  slender  footstalks,  which  give  way  to  every  passing  wind, 
and  are  thus  continually  brought  into  an  extensive  and  ever-changing  gaseous  contact. 
It  is  scarcely  possible  to  conceive  a  more  simple  and  effective  contrivance,  or  one  which 
reaches  more  perfectly  its  destined  end.  To  animals  powers  of  locomotion  are  given, 
with  a  view  of  securing  a  possession  of  food  ;  at  proper  seasons  the  wild  pigeon  comes 
up  from  the  southern  countries,  and  instinctively  flies  thousands  of  miles ;  and  in  the 
case  of  many  beasts,  to  procure  their  prey  seems  to  be  a  principal  cause  of  movement. 
An  animal,  an  oxydating  machine,  is  driven  by  Nature  to  expend  his  powers  of  loco- 


THE  DIGESTION  OF  PLANTS.  21 

motion,  and  go  in  search  of  his  food  ;  a  plant,  a  reducing  apparatus,  has  a  more  dehcate 
duty  to  perform,  and  Nature  herself  becomes  its  minister,  and  offers  it  a  supply  for  all 
its  wants. 

55.  Under  these  circumstances,  therefore,  when  leaves  of  aerial  plants  are  placed  in 
carbonated  water  in  sunshine,  we  can,  so  long  as  their  structure  remains  unimpaired,  ob- 
serve with  a  certain  degree  of  correctness  the  phenomena  which  they  would  exhibit 
under  more  natural  conditions.  By  botanists  and  the  earlier  writers  on  chemistry,  the 
function  discharged  by  the  green  parts  of  plants  is  often  spoken  of  as  a  species  of  res- 
piration, analogous,  to  a  certain  extent,  to  animal  respiration.  This  mistake  originated 
in  those  obscure  physiological  views  of  which  the  old  doctrine  of  vitality  was  the  pro- 
lific parent.  Respiration  is  essentially  an  oxydizing  process,  a  process  of  combustion, 
but  the  part  which  is  played  by  a  vegetable  leaf  is  to  obtain  carbonaceous  matter  from 
air,  and  store  it  up  in  a  solid  form  in  its  various  strictures.  This  solid  matter  thus  ob- 
tained is  the  very  same  which  at  an  after  period,  entering  the  digestive  organs  of  ani- 
mals, is  by  them  subjected  to  assimilatory  processes,  and  finally  becomes  a  part  of  their 
fabric.  The  action  which  is  performed  by  those  green  parts  is  not,  then,  a  respiratory 
action,  but  one  of  digestion. 

56.  We  have  already  said  that  leaves  placed  in  spring  water  in  sunshine  evolve  bub- 
bles of  gas.  A  direct  sunshine  is  not,  however,  absolutely  required;  the  same  experi- 
ment, if  conducted  with  proper  precautions,  can  be  made  to  succeed  with  the  diffused 
skylight.  The  bubbles,  as  they  form,  rise  to  the  top  of  the  water,  and  if  collected  and 
analyzed,  yield,  as  in  the  case  of  water-plants,  two  substances,  oxygen  and  nitrogen;  a 
little  carbonic  acid  is  always  present,  but  that  arises  from  the  peculiar  conditions  under 
which  the  experiment  is  made  (Ap.,  794). 

57.  Thus,  by  the  influence  of  the  sunlight,  organic  matter  is  added  to  vegetable  sys- 
tems, the  action  being  accompanied  by  a  variety  of  chemical  decompositions  and  hiterstitial 
diffusions.  The  substances  arising  are  such  as  are  necessary  for  the  uses  of  the  plant, 
and  in  order  to  distribute  them  to  the  requisite  parts,  mechanical  motion  has  to  take 
place.  This,  in  the  more  highly  organized  plants,  goes  under  the  designation  of  the 
flow  of  the  sap.  The  descending  sap,  like  the  arterial  blood  of  animals,  contains  all 
the  different  compounds  which  are  required  by  the  organized  structure.  We  shall  in 
the  next  chapter  consider  the  causes  which  direct  the  movements  of  this  liquid. 


22      FLOW  OF  THE  SAP  AND  CIRCULATION  OF  BLOOD  DUE  TO  THE  SAME  CAUSE. 


CHAPTER  II. 

ON  THE   MECHANICAL   CAUSE  OF  THE  FLOW  OF   THE  SAP  IN  PLANTS.      IT  IS  DUE  TO  THE 
CARBONIZATION  OF  WATER  IN  THE  LEAVES  BY  THE  LIGHT  OF  THE  SUN. 

Contents  :  The  Floiv  of  Sap  and  Circulation  of  Blood  are  proiaUy  due  to  the  same 
Physical  Cause. — Amount  of  Water  circulating  in  Plants. — Botanical  Theory  of  the 
Flow  of  Sap— fails  for  the  descending  Sap. 

Capillary  Attraction  described. — Elevation  or  Depression  of  Liquids  depends  on  their 
tcetting  or  not  ivetting  the  Tube. — No  Flow  in  an  ordinary  Capillary  Tube. — Con- 
ditions for  producing  a  Flow — such  as  Evaporation,  Decomposition,  and  Solution. — 
Endosmosis  produced  on  these  Princi])les  by  Solution. — Dutrochefs  Experiments. — 
Explanation  of  them. — General  Law  of  these  Movements. — Force  with  which  they 
take  place. — Capillary  Attraction  due  to  Electricity. 

Application  of  these  Principles  to  the  Ascent  of  the  Sap. — Exhausting  Action  of  the 
Leaves. — Cause  of  the  Descent  of  the  Sap. 

The  Light  of  the  Sun  is  the  Cause  of  the  Flow  of  the  Sap  both  in  its  Ascent  and 
Descent. 

58.  In  the  lower  classes  of  plants,  such  as  those  of  which  we  have  been  speaking, 
which  carry  forward  absorbent  processes  on  every  part  of  their  surface,  the  mechanism 
for  nutrition  and  respiration  is  of  the  simplest  character,  and,  as  we  shall  hereafter  see, 
is  nothing  more  than  a  surface  action ;  imbibition,  nutrition,  and  aeration  all  taking 
place  upon  the  same  point.  But  in  any  organized  structure,  as  soon  as  types  of  cen- 
tralization are  adopted,  and  specific  processes  carried  on  in  distant  parts,  the  nutritious 
juice  must  necessarily  pass  from  place  to  place,  and  undergo  in  its  route  proper  chemi- 
cal changes.  In  its  movements  it  must  flow  along  predetermined  channels,  and  take  in 
succession  given  directions.  To  accomplish  this  a  circulatory  apparatus  is  required, 
adapted  in  each  instance,  as  to  form  and  character,  to  each  peculiar  organism.  In  the 
higher  classes  of  animal  life,  this  process  of  movement  passes  under  the  designation  of 
the  circulation  of  the  blood  ;  in  plants,  under  the  designation  of  the  flow  of  the  sap. 

59.  There  is  that  unity  of  plan  in  all  the  works  of  Nature  which  causes  us  at  once 
to  understand  that  in  these  various  mechanisms  the  same  physical  principles  are  resorted 
to ;  that  the  flow  of  the  sap  and  the  circulation  of  the  blood  are  due  to  the  same 
powers.  A  theory  of  such  movements,  therefore,  can  only  be  true  when  the  principles 
which  it  involves  give  at  once  a  clear  account  of  every  case,  of  the  flow  in  plants  and 
in  animals,  and  even  in  every  individual  instance  in  each  of  these  great  classes  of 
organized  beings.  Such  a  theory  should  be  applicable  to  the  movements  in  flowering 
plants,  and  to  all  its  various  modifications  in  the  less  complicated  orders  of  vegetable 
life ;  for  each  particular  instance  it  should  show  why  specific  apparatus  is  required,  and 


AMOUNT  OF  WATER  CIRCULATING  IN  PLANTS.  23 

,%vlij,  in  cryptogamous  plants,  none  whatever  is  necessary;  it  should  show,  among  the 
varieties  of  animal  races,  why  this  or  that  mechanism  is  employed  ;  why  insects  have  no 
heart,  or  why  in  fishes  the  aorta  resemhles  in  mechanism  the  portal  vein  of  the  mam- 
malia. In  those  more  elevated  tribes  whose  functions  require  that  several  circulations 
should  be  simultaneously  carried  on,  it  should  give  a  clear  account  of  the  mechanism 
for  them,  whether  they  be  systemic,  pulmonary,  or  portal.  The  character  of  a  true 
theory  is  its  extensive  and  clear  application  to  all  individual  cases. 

60.  In  this  and  the  following  chapter,  I  propose  to  explain  what  appear  to  be  the 
true  mechanical  principles  of  the  general  circulation  of  organized  beings.  For  some 
years  past  I  have  taught  these  doctrines  to  the  medical  classes  of  this  University,  though 
until  now  they  have  not  appeared  in  print.  Founded  as  they  are  on  principles  strictly 
physical,  they  form  an  important  portion  of  the  views  set  forth  in  this  work.  If  among 
plants  light  is  the  great  agent  of  organization,  electricity  is  the  great  motive  force,  which, 
under  a  specific  modification,  determines  the  movements  of  nutritious  juices.  The  im- 
ponderable agents  are  the  vital  j)rinciple  of  organized  systems. 

61.  It  will  be  perceived  that  the  doctrines  here  given  apply  to  all  cases  ;  they  are 
also  in  harmony  with  physiological  knowledge.  They  embrace  not  only  the  vegetable 
and  animal  world,  but  the  specific  instances  in  each.  In  individual  cases,  they  give  an 
account  of  the  mechanism  of  the  various  circulations,  whether  they  be  systemic,  pulmo- 
nary, or  portal.  They  also  will  be  found  to  apply  to  those  abnormal  cases  which  arise 
in  disease,  and  offer  an  exact  prediction  of  what  should  take  place  in  inflammation  and 
asphyxia. 

62.  First,  therefore,  let  us  take  up  the  consideration  of  the  circulation  of  the  sap  in 
flowering  plants,  and  show  the  physical  conditions  on  which  it  depends.  In  the  follow- 
ing chapter  we  shall  apply  the  same  principles  to  the  circulation  of  the  blood. 

63.  Early  in  the  spring,  in  the  Southern  States,  the  farmers  plant  their  corn — the 
Zea  mays.  In  the  course  of  some  days,  if  the  weather  is  warm  and  favourable,  the  seed 
begins  to  germinate,  and  presently  the  young  plant  appears  above  the  soil.  As  soon  as 
this  has  taken  place,  the  shoot  turns  green,  the  radicles  begin  to  extend  themselves,  and 
growth  rapidly  sets  in.  In  a  few  weeks  the  plant  has  risen  to  an  altitude  of  several 
feet,  and  exposes  its  large  green  leaves  to  the  sun.  By  the  end  of  July,  or  ihe  middle 
of  August,  its  maximum  height  is  attained,  and,  under  favourable  circumstances  of  cul- 
ture and  season,  so  great  is  that  height,  that  a  man  on  horseback  cannot  touch  the  tas- 
sel which  adorns  the  top  of  the  plant  with  a  walking-stick.  All  this  enormous  evolu- 
tion of  organized  structure  has,  in  the  course  of  a  few  days  or  weeks,  originated  from 
a  seed  buried  in  the  ground — a  seed  which  weighed  but  a  few  grains. 

64.  From  the  roots  to  the  top  of  the  plant,  large  quantities  of  fluid  are  constantly 
passing,  and  large  quantities  are  thrown  off  from  the  leaves  by  evaporation.  All  this 
water  is  obtained  entirely  from  the  soil,  and  all  the  carbonaceous  matter  which  con- 
stitutes the  solid  part  is  derived  from  the  atmosphere ;  and,  inasmuch  as  even  the  water 
itself  descended  originally  from  the  air  in  showers  of  rain  or  drops  of  dew,  we  are  justi- 
fied in  saying  that  this  immense  mass  of  organized  matter  is  nothing  more  than  a  por- 
tion of  the  atmosphere,  Avhich  has  been  condensed  and  fashioned  in  a  few  weeks  by  the 
agency  of  the  imponderable  principles. 


24 


BOTANICAL  THEORY  OF  THE  FLOW  OF  SAP. 


65.  In  those  hot  climates,  the  quantity  of  sap  which  flows  in  a  short  space  of  time 
through  the  vessels  of  such  a  plant  is  incredibly  great.  In  the  month  of  April,  1834, 1  cut 
a  vine,  which  was  growing  wild  on  the  edge  of  a  forest  in  Virginia,  asunder  with  one  blow 
of  an  axe  ;  the  cut  surface,  which  was  about  1!  inch  in  diameter,  exhibited  its  open  ves- 
sels, from  which  there  poured  out  an  uninterrupted  stream  of  ascending  sap.  In  the 
course  of  eight  hours  there  was  collected  of  this  fluid  seventy  ounces,  and  this  was 
probably  a  far  less  (juantity  than  would  have  been  raised  under  ordinary  circumstances, 
where  the  leaves  aided  the  spongioles  by  their  exhausting  and  pushing  action,  in  the 
way  which  will  be  presently  explained.  As  all  persons  know,  in  these  warm  climates 
processes  of  vegetation  go  on  with  extraordinary  rapidity,  and  in  summer,  from  the  in- 
tense brilliancy  of  the  sun  and  the  high  temperature,  the  midday  assumes  peculiarities 
which  in  colder  climates  are  never  witnessed.  In  the  forests  of  those  countries,  at  that 
hour,  there  reigns  an  unbroken  silence,  a  period  of  complete  repose.  The  wild  dove, 
which  all  morning  long  has  poured  forth  her  plaintive  note  from  the  top  of  some  with- 
ered pine,  seeks  for  a  shady  covert,  and  plumes  her  feathers  in  the  heat  of  the  day  ; 
the  wanton  squirrel  forsakes  his  gambols,  and  retires  to  his  nest ;  the  turtle  dozes  on 
the  surface  of  the  stagnant  pools.  There  is  not  a  cloud  upon  the  sky,  there  is  not  a 
breath  in  the  air.  The  sunbeams  tremble  upon  the  leaves,  or  sparkle  upon  the  sand, 
or  steal  in  long  gleams  of  hght  across  the  water.  A  profound  silence  reigns  among 
myriads  of  living  things,  inhabitants  of  those  sohtudes  ;  a  silence  only  broken  at  inter- 
vals by  the  rustling  of  the  brown  lizard  in  the  leaves,  or  the  distant  tap  of  the  lazy 
red-headed  woodpecker  upon  souie  hollow  trunk. 

66.  As  we  have  said,  if  at  the  proper  season  of  the  year  the  stem  of  a  plant  be  divi- 
ded, ascending  sap  will  copiously  flow  from  the  extremity  of  the  stump.  If,  moreover, 
the  part  which  has  been  cut  off,  and  which  bears  the  leaves,  has  its  cut  extremity  im- 
mersed in  a  vessel  of  water,  imbibition  of  that  water  will  rapidly  take  place,  and  life 
be  maintained  for  a  time.  From  these  facts,  and  others  of  a  similar  kind,  botanists 
have  had  no  difficulty  in  recognising  two  distinct  sources  of  action  concerned  in  the 
flow  of  the  sap.  They  have  shown  that  the  spongioles  or  extremity  of  the  roots,  which 
apparently  consist  of  a  lax  cellular  or  spongy  tissue,  have  the  quality  of  impelling  the 
sap  upward,  and  hence  it  flows  from  the  extremity  of  a  stem  from  which  the  upper  part 
has  been  cut  off*.  From  the  fact  that  a  branch  on  which  the  leaves  still  remain  pos- 
sesses the  quality  of  removing  water  from  a  vessel  in  which  its  extremity  is  immersed, 
they  have  regarded  those  organs  as  possessing  a  kind  of  suction  power,  due  to  the  evap- 
oration which  takes  place  under  the  influence  of  heat  from  their  superficies.  The  spon- 
gioles, therefore,  drive  the  sap  upward,  and  the  leaves,  by  their  exhaustive  eflbrt,  draw  it. 

67.  But  this  doctrine  is  essentially  defective.  It  furnishes  no  reason  for  the  down- 
ward flow  of  the  sap ;  none  for  the  well-known  fact  that  the  exhausting  action  of  the 
leaves  is  under  the  control  of  light.  To  a  certain  extent,  it  is  true,  an  evaporation  ta- 
king place  from  the  leaves  will  conspire  in  its  result  with  the  movement  of  the  ascend- 
ing sap,  but  it  must  not  be  forgotten  that  it  will  also  antagonize  with  the  movement  of 
that  which  is  descending.  That  there  are  two  seats  of  action  in  the  phenomenon,  the 
spongiole  and  the  leaf,  is  quite  true  ;  but  its  causes  a  e  very  different  from  those  genrr- 


Capillary  attraction  described.  25 

ally  supposed.  The  action  in  the  leaf  and  the  action  in  the  spongiole  is  the  same,  and 
it  is  precisely  an  example  of  what  takes  place  in  the  passage  of  arterial  blood  to  the 
veins  in  the  systemic  circulation  of  the  mammalia. 

68.  It  is  not  worth  while  to  expend  any  space  here  in  refuting  the  old  explanations 
of  these  different  circulatory  movements,  nor  in  detailing  how,  in  the  opinion  of  some 
physiologists,  even  in  the  most  rigid  vegetable  stems,  the  juices  are  propelled  by  alter- 
nate dilatations  and  contractions  of  the  woody  tubes,  much  in  the  same  manner  as  mo- 
tions are  executed  by  the  dorsal  vessels  of  certain  animals.  Nor  need  we  describe  how 
the  vital  principle — that  chimera  of  the  Dark  Ages,  which  has  kept  physiology  in  the 
rear  of  all  other  sciences — can  be  brought  to  give  one  of  its  usual,  and  expeditious,  and 
unsatisfactory  accounts  of  the  phenomenon. 

69.  Without  wasting  time,  therefore,  on  those  futile  explanations,  let  us  pass  at  once 
to  the  philosophical  principles  which  are  involved,  and  show  how  the  doctrines  of 
common  capillary  attraction  are  capable  of  giving  not  only  a  comprehensive,  but 
also  a  beautifully  simple  explanation  of  the  whole  phenomena,  no  matter  whether  they 
are  found  in  flowering  or  in  flowerless  plants,  in  sponges,  or  in  the  mammalia. 

70.  Capillary  attraction,  of  the  physical  cause  of  which  I  shall  presently  speak, 
takes  its  name  from  the  circumstance,  that  if  a  glass  tube  of  small  diameter,  or  even  as 
fine  as  a  hair  {cajnllus),  be  immersed  at  one  end  in  water,  the  water  immediately 
rises  above  its  true  hydrostatic  level  to  an  altitude  which  is  greater  in  proportion  as  the 
tube  is  smaller ;  in  tubes  of  very  narrow  diameter,  such  as  those  here  referred  to,  an 
elevation  of  many  inches  is  without  any  kind  of  difficulty  obtained.  Thus,  in^^.  108, 
if  some  water  be  placed  in  a  cup  or  other  vessel  to  the  height  A  B,  and  there  be  plun- 
ged into  this  water  glass  tubes  such  as  D,  E,  the  water  at  once  spontaneously  rises 
in  those  tubes  to  a  height  which  is  greater  in  proportion  as  the  tube  is  narrower.  In 
E,  therefore,  it  rises  higher  than  in  D. 

71.  But  this  elevation  from  the  true  hydrostatic  level  only  take  place  with  certain 
liquids  in  certain  tubes.  Thus,  with  the  same  glass  tubes,  D  and  E,  if  quicksilver  is 
used  instead  of  water,  so  far  from  there  being  an  elevation,  there  is  an  analogous  de- 
pression. The  liquid  metal  is  forced  down,  as  it  were,  beneath  its  proper  level  to  a 
greater  depth  in  proportion  as  the  tube  is  narrower.  And  that  this  depends  on  the 
chemical  relation  which  subsists  between  the  liquid  employed  and  the  substance  of 
which  the  tube  consists,  is  clearly  shown  by  smearing  the  interior  of  a  glass  tube  with 
tallow  or  oil,  and  then  immersing  its  end  in  water.  The  water,  under  these  circum- 
stances, so  far  from  rising,  is,  like  quicksilver,  depressed. 

72.  The  physical  law  under  which  these  elevations  and  depressions  takes  place  is 
very  simple,  and  important  to  be  remembered.  If  a  liquid  can  wet  the  surface  of  a 
solid,  it  will  rise  in  a  tube  formed  of  that  substance ;  but  if  a  hquid  cannot  wet  a  solid, 
it  will  be  depressed  below  its  true  level  in  a  tube  formed  of  that  substance. 

73.  Suppose,  now,  we  had  a  glass  tube  immersed  in  water,  a  tube  of  such  diameter 
that  it  could  cause  the  water  to  rise  to  the  altitude  of  twelve  inches,  and  the  tube  be 
broken  off  so  that  it  is  only  six  inches  long.  The  theory  of  capillary  attraction,  and 
also  direct  experiment,  show  that  under  these  circumstances  the  water  will  rise  to  the 

D 


.26 


CONDITIONS  FOR  PRODUCING  A  CONTINUOUS  FLOW. 


top  of  the  tube,  hut  will  not  jiow  over,  as  one  might  perhaps  have  expected.  In  an  or- 
dinary capillary  tube,  therefore,  nothing  like  a  constant  flow  can  take  place,  but  the 
liquid  having  attained  its  highest  possible  elevation,  remains  there. 

74.  Tiutt  a  continuous  flow  should  take  place,  all  that  is  necessary  is,  by  any  proper 
means,  either  by  evaporation,  chemical  action,  or  other  processes,  to  remove  away  the 
superficial  portions  of  the  elevated  liquid  when  they  stand  at  the  extremity  of  the  tube. 
An  illustration  will  show  how  this  is  accomplished.  The  wick  of  a  lamp  is  nothing 
more  than  an  extensive  system  of  capillary  tubes,  tubes  which  are  formed  by  the  juxta- 
position of  the  cotton  fibres.  As  common  experience  satisfies  us,  such  a  wick  may  be 
immersed  in  a  reservoir  of  oil  for  months,  or  even  years,  without  any  sensible  portion 
of  that  liquid  being  removed ;  but  if  the  lamp  is  lighted,  the  process  of  combustion  dis- 
sipating the  oil  as  fast  as  it  reaches  the  upper  portion  of  the  wick,  fresh  quantities  are 
furnished  from  beneath,  and  a  continual  flow  takes  place  until  all  the  oil  is  gone.  So 
also  in  a  spirit  lamp,  as  long  as  the  extinguisher  is  over  the  wick,  and  no  evaporation 
of  the  alcohol  can  take  place,  there  is  no  flow  ;  but  the  moment  the  extinguisher  is  remo- 
ved, so  that  evaporation  into  the  atmosphere  can  be  accomplished,  there  is  a  constant 
upward  flow  until  the  alcohol  is  dissipated.  These  are  results  which  have  been  long 
recognised. 

75.  From  these  elementary  considerations,  therefore,  it  is  clear,  that  although,  in  the 
ordinary  use  of  a  capillary  tube,  continuous  movement  along  it  is  not  witnessed,  that 
movement  readily  sets  in  as  soon  as  the  proper  conditions  are  fulfilled.  In  the  two 
cases  we  have  used  as  illustrations,  combustion  in  the  one,  and  evaporation  in  the  other, 
gave  rise  to  a  continuous  motion. 

76.  In  the  same  way  a  variety  of  other  ordinary  causes  may  produce  these  move- 
ments. Suppose,^o-.  109,  we  had  a  vessel  A,  containing  water,  and  another  vessel  B, 
containing  alcohol,  and  between  them  a  narrow  capillary  tube,  C,  passing.  Let  us 
farther  suppose  this  tube  C  to  be  previously  filled  with  water.  At  its  extremity  which 
opens  into  the  vessel  B,  containing  alcohol,  it  is  clear  that  the  water  will  be  brought 
in  contact  with  the  alcohol,  but  in  this  liquid  water  is  soluble  ;  and,  therefore,  as  fast 
as  that  water  can  be  presented,  the  alcohol  will  dissolve  it,  take  it  up,  and  remove  it 
away.  It  is  plain,  therefore,  that  there  will  be  a  constant  flow  of  water  from  the  ves- 
sel A  to  the  vessel  B,  a  flow  which  arises  from  the  circumstance  that  the  water  is  dis- 
solved from  the  end  of  the  tube  as  fast  as  it  presents  itself,  by  the  affinity  of  the  alco- 
Uol  for  it.  This,  therefore,  is  nothing  more  than  a  repetition,  under  another  form,  of  the 
cases  already  used  above  (74)  as  illustrations. 

76.  But  an  attentive  consideration  of  the  facts  will  convince  us  that  the  affinity  thus 
existing  between  the  two  liquids,  and  which  thus  gives  rise  to  a  flow  from  A  to  B, 
ought,  after  a  short  time,  to  cause  portions  of  the  alcohol  to  find  their  way  through  the 
water  in  the  tube  C,  and  present  themselves  in  a  certain  quantity  at  its  entrance  into 
the  vessel  A  ;  there,  in  their  turn,  they  are  exposed  at  once  to  the  pure  water,  w  hich 
takes  up  and  dissolves  alcohol  just  in  the  same  way  that  alcohol  takes  up  and  dissolves 
it.  There  must,  therefore,  be  a  constant  flow  of  alcohol,  from  B  to  A,  along  the  tube, 
for  the  very  same  reason  that  there  is  a  simultaneously  constant  flow  of  water  frotn  A 
to  B,  in  the  contrary  direction. 


/ 


ENDOSMOSES  PRODUCED  ON  THESE  PRINCIPLES  BY  SOLUTION.  27 

77.  The  general  result  of  these  considerations  proves,  that  if  two  different  liquids, 
which  can  dissolve  one  another,  communicate  through  a  tube  which  both  can  wet, 
both  of  them  will  flow  through  that  tube  contemporaneously,  the  one  passing  in  one, 
and  the  other  in  the  opposite  direction ;  and  it  is  plain  that  all  this  is  nothing  more 
than  a  simple  case  of  common  capillary  attraction. 

7S.  What  happens  through  one  will,  under  similar  circumstances,  happen  through 
one  hundred  or  any  number  of  tubes.  If,  therefore,  instead  of  a  single  tube  C,  a  great 
number  of  tubes  were  made  to  communicate  between  A  and  B,  they  would  all  act  aHke, 
and  through  them  the  two  liquids  would  simultaneously  pass  in  opposite  directions. 

79.  It  is  also  obvious  that  the  shorter  we  make  the  tube  C,  or  the  supposed  collec- 
tion of  tubes,  the  more  readily  will  the  flow  take  place,  because  the  vessels  A  and  B 
are  then  made  to  communicate  through  a  shorter  obstacle.  If,  therefore,  we  take  a 
box,  A  B,^^.  110,  and  divide  it  water-tight  by  any  porous  obstacle  C,  such  as  a  piece 
of  paper,  or  bladder,  or  porous  earthenware,  &c.,  which  substances  may  be  regarded  as 
consisting  of  a  congeries  of  very  short  tubes,  their  pores  answering  to  such  short  tubes, 
and  in  the  compartment  A  place  water,  and  in  B  alcohol,  through  the  intervening  bar- 
rier interchange  will  take  place  ;  and  if  there  be  no  leakage,  and  one  liquid  passes  more 
rapidly  in  its  course  than  the  other,  not  only  will  the  interchange  we  have  been  de- 
scribing take  place,  but  there  will  be  also  an  accumulation  of  liquid  on  one  side  of  the 
barrier  C,  and  a  diminution  of  it  on  the  other. 

80.  All  this  is  irrespective  of  the  shape  or  form  of  the  vessels,  which  may  be  cubical, 
or  round,  or  of  any  other  figure.  The  essential  conditions  for  action  are  to  have  two 
liquids,  which  have  an  affinity  for  one  another,  placed  on  opposite  sides  of  a  pervious 
obstacle  or  porous  barrier,  which  both  of  them  can  wet.  Movement  will  then  take 
place  in  opposite  directions ;  and,  if  one  liquid  flows  more  rapidly  than  the  other,  there 
will  be  an  accumulation  on  that  side  of  the  barrier  to  which  it  goes,  and  a  deficiency 
on  the  other. 

81.  M.  DuTROCHET  took  a  bladder,  and,  filling  it  with  alcohol,  tied  the  mouth  of  it 
tightly,  so  that  none  of  the  liquid  could  escape.  He  then  placed  it  in  a  vessel  of  wa- 
ter, and  found  that  the  alcoliol  came  out  of  the  bladder  into  the  water,  and  the  water 
passed  through  the  bladder  into  the  alcohol,  and,  inasmuch  as  the  water  flowed  more 
rapidly  than  the  alcohol,  there  was  a  constant  accumulation  within  the  bladder,  dis- 
tending it ;  an  accumulation  taking  place  with  sufficient  force  to  burst  it  open,  provi- 
ded the  experiment  was  continued  long  enough. 

82.  To  these  phenomena  M.  Dutrochet  gave  the  name  of  Endosmose  and  Exos- 
mose,  the  former  in  allusion  to  the  current  flowing  inward,  the  latter  to  that  flowing 
outward.  Physiologists  were  pleased  with  these  sonorous  designations,  which  ever 
since  have  been  used  in  the  books.  Some  supposed  that  the  force  thus  exerted  by  a 
bladder  was  due  to  the  remains  of  vitality  still  existing  in  it,  arising  from  its  organization. 
It  is,  however,  as  we  have  seen,  one  of  the  ordinary  cases  of  common  capillary  attrac- 
tion, with  which  the  vital  force  has  no  more  connexion  than  its  kindred  principle 
Phlogiston. 

83.  The  rise  or  depression  of  a  liquid  in  a  capillary  tube  is  determined  by  its  quali- 


■58 


GENERAL  LAW  OF  THESE  MOVEMENTS. 


ty  of  wetting  or  not  wetting  the  surface  of  that  tube.  Of  two  liquids  in  a  given  tube, 
that  will  rise  highest  which  will  wet  the  tube  most  perfectly.  And,  therefore,  we  can 
see  in  these  movements,  that  if  two  liquids  be  placed  on  opposite  sides  of  a  porous  sys- 
tem, or  at  the  opposite  ends  of  a  capillary  tube,  which  is  wetted  by  one  more  perfectly 
than  by  the  other,  that  one  which  exerts  the  most  energetic  action  will  flow  fastest.  If 
a  piece  of  bladder  be  soaked  in  water  and  in  alcohol,  it  will  be  readily  seen  that  the 
former  acts  more  powerfully  on  it,  giving  it  greater  flexibility  and  translucency,  and 
having  a  stronger  affinity  for  its  tissues.  For  these  reasons,  if  a  mixture  of  water  and 
alcohol  be  tied  up  in  a  bladder,  as  is  well  known,  the  water  will  soak  out  and  evapo- 
rate away,  but  the  alcohol  will  be  retained.  And  for  the  very  same  reason,  when  these 
two  liquids  are  placed  on  opposite  sides  of  such  a  porous  body,  the  water  moves  fast- 
est through  it,  as  in  the  experiment  of  Dutrochet  (81). 

84.  From  these  simple  principles  we  deduce  the  following  important  law — important, 
because  it  gives  us  at  once  a  clear  explanation  of  the  rise  of  sap  in  trees,  and  a  beau- 
tiful exposition  of  the  true  cause  of  the  circulation  of  the  blood  :  When  two  different 
liquids  are  brought  in  contact  in  a  porous  solid,  which  is  wetted  hy  both,  hut  hy  them 
unequally,  that  one  %ohich  has  the  greatest  affinity  for  the  solid,  or  ivhich  wets  it  most 
perfectly,  will  pass  most  rapidly  through  it,  and  may  even  drive  the  other  entirely  he- 
fore  it. 

85.  This  passage  is  not  accomplished  with  an  insignificant  force.  Direct  experi- 
ment shows  that  (Ap.,  145)  water  will  thus  pass  into  alcohol  through  a  pervious  mem- 
brane with  a  force  equal  to  the  pressure  of  nearly  two  atmospheres.  Nor  is  it  alone 
between  liquids  that  the  phenomenon  takes  place ;  it  is  exhibited  also  by  gases. 
Here,  again,  the  force  with  which  the  movement  has  been  accomplished  is  surprisingly 
great.  Sulphurous  acid  will  pass  (Ap.,  158)  into  atmospheric  air  against  a  pressure  of 
one  hundred  and  ten  pounds  on  the  square  inch,  and  sulphuretted  hydrogen  will  move 
through  a  membrane  with  a  force  that  is  superior  to  a  pressure  of  twenty-four  atmo- 
spheres (Ap.,  162).  As  a  mechanical  agent,  capillary  attraction,  therefore,  is  fully  able 
to  overcome  any  of  the  resistances  which  it  has  to  encounter  in  elevating  sap  to  the 
tops  of  the  loftiest  trees,  or  driving  blood  from  the  remotest  parts  of  the  largest  ani- 
mals. 

86.  In  the  Appendix  I  have  given  (Chap.  V.)  an  account  of  the  physical  principles 
on  which  capillary  attraction  itself  depends.  It  is  needful,  therefore,  in  this  place,  only 
to  sum  up  the  evidence  there  brought  forward,  that  it  is  nothing  more  than  a  manifesta- 
tion of  electricity,  and  that  all  capillary  phenomena  are  cases  of  electrical  attraction. 
If  we  take  a  piece  of  flat  glass,  and  place  it  on  the  surface  of  some  mercury,  the  glass  is 
held  to  the  metal  with  a  considerable  degree  of  force,  so  that  it  requires  some  exertion 
to  separate  them.  When  this  is  done,  and  the  electric  condition  of  the  mercury  and 
glass  respectively  examined,  the  one  is  found  to  be  positively  and  the  other  negatively 
electrified.  They  must  therefore  attract  each  other  (Ap.,  116)  ;  and  it  is  owing  to  this 
electric  excitement,  which  always  takes  place  upon  the  contact  of  bodies,  that  all  the 
phenomena  of  capillary  attraction  are  due.  Thus,  if  two  pieces  of  plate  glass  are  brought 
in  contact,  they  are  found  to  cohere,  because  the  one  is  positive  and  the  other  negative, 


/ 


GENERAL  LAW  01"  I-JIESE  MOVEMENTS. 


29 


as  may  be  proved  by  examining  them  with  an  electrometer.  Even  in  tliose  cases  in 
which  attractive  forces  are  developed  without  an  apparent  electric  disturbance,  these 
principles  apply.  Two  leadeu  bullets,  which  have  been  brought  (Ap.,  12 G)  into  con- 
tact with  one  another,  cohere  strongly,  but  we  are  not  able  to  show  the  development 
of  the  separate  electricities  on  each,  because  of  their  high  conducting  power.  So,  too, 
when  a  piece  of  glass  is  laid  on  the  surface  of  some  water,  and,  on  being  lifted  olf,  is 
carried  to  the  electroscope;  no  development  can  be  detected,  and  the  reason  is  obvious, 
for  there  has  been  only  an  apparent,  and  not  a  true  separation  of  the  liquid  and  the 
glass  from  each  other ;  the  particles  of  the  liquid  have  been  simply  lorn  apart,  and  have 
not  been  separated  from  the  glass. 

87.  Referring,  therefore,  to  that  chapter  for  the  proof  that  capillary  attraction  origi- 
nates in  electric  disturbance,  it  is  sufficient  for  our  present  purpose  to  remark,  that  it  is 
therefore  due  to  the  very  same  cause  as  chemical  affinity  itself  The  quality  which 
liquids  possess  of  wetting  or  not  wetting  the  surface  of  solid  bodies  is,  therefore,  nothing 
more  than  an  indication  of  the  affinity  which  is  between  them.  Quicksilver  will  not 
wet  glass,  because  they  have  little  affinity,  but  it  will  wet  a  surface  of  gold  or  of  tin  with 
facility,  because  its  affinity  for  those  bodies  is  energetic.  These  observations,  which 
appear  so  simple,  have  very  important  applications ;  the  intensity  of  affinity  between  a 
given  liquid  and  a  solid  with  which  it  is  brought  in  contact  determines  their  capillary 
relations,  and  thereby  determines,  also,  the  phenomena  of  movement.  No  farther  proof 
of  this  importance  is,  perhaps,  required  than  the  result  to  which  we  shall  be  presently 
led,  thai  even  in  man  the  circulation  of  the  blood  is  caused  by  the  oxydating  action  of  tltat 
liquid  on  the  solid  structures  with  which  it  is  brought  in  contact. 

88.  Let  us,  therefore,  finally  remember  that  the  explanation  of  the  circulation  of  nu- 
tritious juices,  both  in  the  vegetable  and  the  animal  kingdom,  rests  upon  this  simple 
physical  principle,  that  if  two  liquids  communicate  ivith  one  another  in  a  cajnllary  tube, 
or  in  a  jwrous  or  j^afenchy/natous  structure,  and  have  for  that  tube  or  strvcture  different 
chemical  affinities,  movement  will  ensue  ;  that  liquid  which  has  the  most  energetic  affin- 
ity will  mom  with  the  greatest  velocity,  and  may  even  drive  the  other  fluid  entirely  be- 
fore it ;  that  this  is  due  to  common  capillary  attraction,  which,  in  its  turn,  is  due  to 
electric  excitement. 

89.  These  things  being  understood,  let  us  proceed  now  to  apply  our  principle  (84) 
to  the  cases  in  hand,  and  commence  with  giving  the  theory  of  the  flow  of  sap  in  plants, 

90.  The  liquid  of  which  the  ascending  sap  is  constituted  is  derived  from  the  ground 
by  the  action  of  the  spongioles,  and  consists  of  water  holding  in  solution  the  different 
saline  bodies  which  are  necessary  to  the  plant,  along  with  carbonic  acid,  &c.  This 
compound  fluid  passes  upward  by  the  woody  fibre  and  ducts  of  the  alburnum,  niaking 
its  way  to  the  leaf,  on  the  upper  surface  of  which,  in  common  cases,  a  change  in  its 
chemical  constitution  occurs  through  the  influence  of  the  sunlight.  It  obtains  a  cer- 
tain quantity  ol  carbon,  and  from  being  a  thin  watery  solution,  becomes  much  concen- 
trated, and  gains  the  under  face  of  the  leaf.  This  elaborated  sap,  or  latex,  as  it  is  fre- 
quently called,  returns  now  to  the  bark,  and  descends  through  its  cellular  tissue  and 
iDter-cellular  spaces,  finding  its  way  by  the  route  of  the  medullary  rays  to  all  parts  of 


30' 


APPLICATION  OF  THESE  PRINCIPLES  TO  THE  ASCENT  OF  THE  SAP. 


the  plant.  During  its  descent  the  different  vegetable  principles  necessary  for  the  econ- 
omy of  the  plant  are  removed  from  it,  and  a  certain  quantity  goes  down  to  the  roots, 
partly  to  aid  in  their  growth,  and  partly  to  throw  new  qualities  of  ascending  sap  into 
the  tree.  In  this  descent,  the  elaborated  sap  moves  through  a  system  of  vessels  which 
anastomose  with  one  another,  in  the  same  manner  as  the  capillary  vessels  of  animals. 
These  tubes  go  under  the  technical  name  of  laticiferous  tubes. 

91.  We  see,  therefore,  from  this  description,  that  there  are  two  points  of  this  circula- 
tion which  require  attentive  consideration — the  spongiole  and  the  leaf  The  spongi- 
oles  are  nothing  but  the  young  succulent  extremities  of  the  roots,  which  have  been  re- 
cently formed  from  portions  of  the  descending  sap,  but  that  sap  is  itself  a  species  of 
mucilaginous  solution.  Precisely,  therefore,  as  water  will  pass  through  the  tissue  of  a 
bladder,  the  interior  of  which  is  filled  with  gum-water,  so  will  moisture  from  the  ground 
flow  through  the  spongiole.  There  is  no  difficulty  in  accounting  for  the  rise  of  the  as- 
cending sap  on  the  principles  of  capillary  attraction,  and,  indeed,  this  is  the  explana- 
tion now  generally  received  by  vegetable  physiologists. 

92.  While,  therefore,  those  philosophers  have  freely  admitted  the  applicability  of  this 
principle,  under  the  name  of  endosmosis,  to  the  explanation  of  the  ascent  of  the  sap, 
they  have  attributed  to  it,  as  an  aid,  a  force  which  comes  into  operation  only  in  an 
incidental  way.  This  is  the  exhausting  action  of  the  leaf  But  it  is  probable  that  this 
force  takes  little  or  no  part  in  the  movement  of  the  sap,  for  any  tendency  to  a  vacuum 
occurring  in  the  leaves  of  a  tree  would  cause  those  structures  to  collapse,  and  not 
eventuate  in  exerting  suction  power  on  the  rising  fluid.  It  is  true,  that  if  we  take  (67) 
a  branch  covered  with  its  foliage,  and  dip  its  cut  extremity  into  water,  imbibition  of  that 
water  will  rapidly  take  place ;  but  the  phenomenon  is  certainly  not  due,  as  is  ordinarily 
supposed  by  botanists,  to  the  exhausting  action  of  the  leaf,  but  to  a  very  different  cause. 
The  exhaustion  is  only  an  incidental  affair. 

93.  Guided  now  by  the  principle  we  have  laid  down  (88),  let  us  predict  what  must 
be  the  necessary  action  of  the  leaf  The  ascending  sap,  which  we  will  suppose,  for 
simplicity's  sake,  to  be  water,  rises  to  the  upper  face  of  the  leaf.  It  there  obtains  car- 
bonic acid  gas  from  the  air,  of  which  the  sunlight  effects  the  decomposition,  the  result- 
ing action  being  a  change  from  water  to  a  mucilaginous  solution.  In  the  tissue  of  the 
leaf  we  have,  therefore,  two  fluids  engaged,  warter  and  a  mucilaginous  solution ;  and 
what  must  of  necessity  be  the  result?  The  water  will  drive  the  mucilaginous  solution 
before  it  (84-88),  and  force  it  back,  along  its  proper  vessels,  into  the  stem.  The  imbi- 
bition, therefore,  that  we  perceived  when  a  branch  is  dipped  in  a  vessel  of  water,  does 
not  arise,  as  the  botanists  say,  from  evaporation  taking  place  on  the  leaf,  but  it  comes 
from  the  capillary  reaction  which  is  going  on  in  the  leaf  between  the  water,  which  is 
then  presented  as  ascending  sap,  and  the  mucilaginous  solution  which  has  been  formed 
by  the  light  of  the  sun.  Evaporation,  it  is  true,  takes  place,  and  comes  into  operation, 
as  I  have  said,  in  an  incidental  way,  but  the  proper  force  which  gives  origin  to  the 
whole  phenomenon  is  the  capillary  action  which  is  going  on  in  the  way  just  described. 

94.  It  is  the  imperfection  of  the  principle  on  which  they  were  relying — the  exhaust- 
ing action  of  the  leaf — that  has  caused  botanists  to  look  upon  the  descent  of  the  sap 


I 

CAUsi:  or  'i'hk  descent  of  the  sap.  31- 

as  such  a  mysterious  affair,  for  which  they  could  offer  no  explanation.  Physiologists 
have  here  stepped  in  whh  their  phantom  vitality,  and  have  explained  the  descent  of 
the  elaborated  sap  on  visionary  hypotheses,  that  it  was  alive,  or  had  obtained  some 
vital  qualities.  It  had  long  been  perceived  that  gravitation  could  have  little  or  nothing 
to  do  with  the  motion,  for  the  descending  sap  flows  upward  in  a  dependent  branch. 

95.  What,  then,  is  the  reason  that  the  light  of  the  sun  controls  the  rapidity  of  imbi- 
bition— the  speed  with  which  the  ascending  current  comes  1  Because  it  controls  the 
amount  of  carbonic  acid  gas  which  is  reduced,  and,  therefore,  the  amount  of  elaborated 
sap  that  is  foruied.  Why  is  it  that  the  flow  from  the  roots  diminishes  when  changes 
are  befalling  the  leaves,  and  why  does  it  stop  in  the  winter  1  Because  the  mucilagi- 
nous solution  which  is  made  by  the  light  diminishes  in  quantity,  or  ceases  to  be  formed 
altogether.  How  is  it  that,  when  parts  are  rapidly  developing,  the  latex  moves  fastest, 
and  the  ascending  sap  comes  with  most  force  ?  Because  the  consumption  and  the  con- 
sequent formation  of  the  mucilaginous  matter  are  then  at  a  maximum. 

96.  We  see,  therefore,  that  the  two  sources  of  force  in  a  flowering  plant,  the  spon- 
giole  and  the  leaf,  derive  their  power  from  ordinary  physical  principles.  And  these 
considerations  also  furnish  us  with  another  instance  of  that  unity  of  plan  so  often  met 
with  in  the  works  of  Nature;  the  same  law  which  determines  the  action  of  the  spon- 
giole  determines  also  the  action  of  the  leaf  The  same  idea  is  concerned  in  throwing 
the  sap  upward  into  the  stem,  and  forcing  it  down  again  from  the  leaf  And  the  rays 
of  the  sun,  which,  by  forming  that  mucilaginous  body,  gives  rise  to  these  concurrent 
and  harmonious  actions,  equally  set  in  operation  the  tissues  of  the  leaf  which  is  freely 
exposed  to  their  influence,  and  the  absorbing  mechanism  of  the  spongiole,  which  is 
buried,  perhaps,  many  feet  deep  in  the  ground. 

97.  Whatever  has  been  here  said  respecting  the  movements  of  the  nutritious  juice 
in  exogenous  plants,  applies  also  to  the  case  of  endogens,  the  essential  mechanism  be- 
ing the  same  in  both  instances. 

98.  It  has  been  clearly  established,  by  the  researches  of  comparative  anatomists,  that 
the  presence  of  a  circulating  machinery  is  determined  by  the  centralization  of  the  nu- 
tritive and  respiratory  apparatus.  In  exogenous  and  endogenous  plants,  from  the  cir- 
cumstance that  the  liquid  and  solid  material  are  introduced  at  distant  points,  the  one 
through  the  root,  the  other  through  the  leaf,  channels  of  communication  from  one 
to  the  other,  and,  indeed,  to  every  part,  are  required,  and  hence  the  introduction  of  a 
circulatory  apparatus.  In  lower  tribes  of  vegetable  life,  where  this  separation  of  func- 
tion does  not  exist,  the  circulatory  mechanism  is  correspondingly  absent ;  the  sea-weeds 
absorb  on  their  whole  surface,  and  nutrition  is  directly  carried  forward  at  the  points  of 
reception.  In  lichens  there  is  the  first  appearance  of  a  transfusory  mechanism,  arising 
from  the  circumstance  that  on  those  parts  which  are  shaded  from  the  light  absorption 
most  rapidly  takes  place ;  here,  probably,  however,  the  channels  of  movement  are  the 
interspaces  between  the  cells,  and  the  cause,  simple  capillary  attraction.  In  mush- 
rooms there  is  a  more  close  approximation  to  the  mechanism  more  fully  developed  in 
the  higher  plants,  for  in  them  the  rootlets  absorb  nutrient  matter  from  the  soil,  from 
which  it  passes,  by  capillary  action,  to  every  part  of  the  system. 


32 


THE  LIGHT  OF  THE  SUN  IS  THE  CAUSE  OF  THE  FLOW  OF  THE  SAP. 


99.  Thus,  in  the  vegetable  world,  if  we  commence  our  examination  with  the  lowesl 
orders  of  cellular  plants,  and  pass  successively  through  the  mosses  and  ferns  to  the 
flowering  plants,  we  see  the  same  physical  cause  in  constant  operation.  They  all  ob- 
tain their  liquid  and  their  solid  food  through  the  agency  of  capillary  attraction — the 
former  from  the  soil,  and  the  latter,  in  higher  tribes,  from  the  air.  It  is,  moreover,  in 
virtue  of  its  action  as  a  capillary  mass,  that  the  ground  becomes  uniformly  moist ; 
and  hence,  through  the  operation  of  the  same  power,  fluid  matter  is  first  brought  to  the 
plant,  and  then  distributed  through  it.  It  is  in  virtue  of  its  capillary  diffusion  through 
atmospheric  air  that  carbonic  acid  gas  is  brought  in  contact  with  the  leaves  ;  and  when 
there  reduced,  it  is  through  the  same  force  that  the  carbon  is  distributed  to  the  grow- 
ing parts.  Of  the  energy  of  this  force  there  is  abundant  evidence,  not  only  that  it  can 
cause  the  gradual  percolation  of  juices  through  small  structures,  such  as  the  minutest 
moss,  but  also  lift  the  sap  to  the  tops  of  the  highest  forest  trees,  and,  indeed,  far  higher 
were  it  necessary,  and  drive  it  back  again  to  the  roots,  irrespective  of  the  agency  of 
gravity.  Even  gaseous  substances,  as  is  shown  experimentally  in  the  Appendix  (Ch. 
XL),  pass  into  one  another  with  a  force  greater  than  the  pressure  of  a  column  of  wa- 
ter seven  hundred  feet  high  ;  so  that,  to  elevate  the  sap  in  a  tree,  or  to  drive  the  blood 
in  an  animal,  is  an  insignificant  demand  on  the  energy  which  this  force  could  put  forth. 

100.  In  conclusion,  we  here  again  remark  the  influence  which  the  imponderable 
principles  exert  in  directing  all  kinds  of  movements  on  the  surface  of  our  globe.  The 
sap  rises  in  a  tree  because  the  sun  shines ;  it  is  the  light  of  that  central  orb  which  pro- 
duces even  these  movements  in  plants.  Indirectly,  it  is  true,  chemical  affinities  or  elec- 
trical agencies  are  brought  into  operation,  but  the  prime  mover  of  the  machine  is  the 
light,  which  produces  a  mucilaginous  body,  which  is  different  in  composition  in  differ- 
ent plants,  and  which  constitutes  their  proper  juices. 

101.  The  cause  of  the  movement  of  the  sap  in  flowering  plants — hath  of  the  rise  of 
the  crude  sap  upward,  and  of  the  descent  of  the  elaborated  sap  downward — is  the  light 
of  the  sun,  lohich  effects  the  decomposition  of  carbonic  acid  gas. 


ANCIENT  THEORY  OF  THE  CIRCULATION. 


33 


CHAPTER  III. 

ON   THE    MECHANICAL   CAUSE    OF   THE   CIRCULATION   OF   THE  BLOOD. 

Contents  :  Ancient  Theory. — Description  of  the  Systemic,  Pulmonary,  and  Portal  Cir- 
culation.— General  Law  of  Movement. — Capillary  Relations  of  Arterial  and  Venous 
Blood  to  the  Tissues. —  The  Systemic  Circulation  is  due  to  the  Deoxydation  of  Arte- 
rial Blood,  and  its  Direction  is  therefore  from  the  Artery  to  the  Vein. 

Pulmonary  Circulation. — Capillary  Relations  of  Arterial  and  Venous  Blood  to  Atmo- 
spheric Oxygen. — Pulmonary  Circulation  is  due  to  the  Oxydation  of  Venous  Blood, 
and  its  Direction  is  therefore  from  the  Venous  to  the  Arterial  Side. —  Uses  and  Action 
of  the  Heart. 

Portal  Circulation. — Capillary  Relations  of  Arterial,  Portal,  and  Venous  Blood  to  the 

Liver. —  Three  Sources  of  Force  in  conducting  the  Portal  Circulation. 
Action  in  Asphyxia. — Case  of  obstructed  Trachea. 

102.  Let  us  now  proceed  to  inquire  how  the  physical  principles  which  have  guided 
us,  in  the  preceding  chapter,  in  determining  the  cause  of  the  flow  of  sap,  apply  to  the 
more  interesting  case  of  the  circulation  of  the  blood  in  the  higher  aniuials. 

103.  The  popular  explanation  which  is  given  of  the  circulation  of  the  blood  in  man, 
refers  to  the  heart  as  the  prime  mover  of  the  mechanism.  This  central  organ  of  im- 
pulse is  devoted  to  a  double  purpose.  It  has  to  throw  the  blood  through  the  channel 
of  the  arteries  to  every  part  of  the  system,  and,  receiving  it  back  again  by  the  veins, 
has  to  throw  it  to  the  lungs,  in  which  it  must  be  submitted  to  the  vivifying  influence 
of  the  air  before  it  can  again  be  restored  to  the  system  to  be  used  for  the  general  pur- 
poses of  the  economy.  In  order  to  enable  it  to  discharge  this  task,  it  is  furnished  with 
an  appropriate  valvular  and  tubular  arrangement,  and,  at  specific  periods,  contracts  and 
dilates,  for  the  purpose  of  ejecting  or  sucking  up  the  circulating  liquid.  In  the  opin- 
ion of  the  older  physiologists,  these  periodic  motions  take  place  either  directly,  for  the 
reason  that  the  heart  is  alive,  or,  as  some  of  them  have  supposed,  through  the  myste- 
rious agency  of  the  cerebro-spinal  axis. 

104.  The  blood,  thus  alternately  driven  from  and  drawn  to  this  centre  of  action,  is, 
in  this  view,  a  living  fluid,  possessed  of  a  great  many  extraordinary  properties.  A  por- 
tion of  it  extracted  from  the  system  by  any  of  the  ordinary  processes  of  phlebotomy, 
soon  coagulates  and  dies.  In  what  this  change  consists,  this  distinction  between  the 
living  and  dead  blood,  does  not  so  plainly  appear.  Moreover,  the  ancient  physiologists 
imputed  a  variety  of  other  equally  important  offices  to  the  heart;  they  regarded  it  as  the 
seat  of  the  passions,  such  as  love,  and  held  it  accountable  for  the  various  deeds  in  which  its 
possessor  was  concerned,  a  moral  accountability  for  good  and  evil.  These  philosophical 
doctrines  have,  to  a  certain  extent,  become  interwoven  in  common  speech,  and  we  now 
often  use  them  without  attaching  any  strict  signification  to  them.    Anatomical  and  me- 

E 


34        DESCRIPTION  OF  THE  SYSTEMIC,  PULMONARY,  AND  PORTAL  CIRCULATION. 


chanical  considerations  might,  perhaps,  lead  us  to  infer  that  these  ancient  views  are  not 
altogether  correct.  There  does  not  seem  much  consonance  between  mitral,  tricuspid, 
and  semihinar  valves,  and  affections  or  passions,  or  the  responsibilities  for  good  and  evil. 
A  pump  may  be  a  very  curious  and  ingenious  piece  of  mechanism,  but  surely  we  can- 
not regard  it  as  a  morally  accountable  agent. 

105.  For  such  reasons,  therefore,  modern  physiologists  are  disposed  to  cast  aside  al- 
together these  ancient  views,  and  look  upon  the  heart  as  an  ordinary  but  beautiful  speci- 
men of  hydrauhc  contrivance,  in  the  same  way  as  they  look  upon  the  eye  as  an  opti- 
cal apparatus. 

106.  In  man  there  are  three  prominent  varieties  of  circulation  :  the  Systemic,  the 
Pulmonary,  and  the  Portal.  It  is  necessary  that  we  should  understand  the  nature  of 
each: 

1st.  The  Systemic. — Arterial  blood,  which  has  been  brought  from  the  lungs  by  the 
pulmonary  veins  into  the  left  auricle,  is  forced  by  its  contraction  into  the  left  ventricle, 
the  contraction  of  which  drives  it  into  the  aorta.  By  this  it  is  distributed  to  all  parts 
of  the  system,  the  arrangement  of  arterial  tubes  becoming  smaller  and  smaller  in  diam- 
eter until  they  degenerate  into  mere  capillaries,  from  which  it  finds  its  way  into  the 
ascending  and  descending  vena  cava,  the  terminal  tubelets  of  which  collect  it,  and  it  is 
returned  to  the  heart  by  the  right  auricle  during  its  dilatation.  In  the  systemic  circu- 
lation, therefore,  the  blood  leaves  the  left  ventricle  as  arterial  blood,  is  finally  distrib- 
uted by  the  capillary  arteries,  undergoes,  on  its  passage  into  the  capillary  veins,  a  chem- 
ical change,  gives  off  oxygen,  from  being  bright  red  it  turns  dark,  and,  becoming  venous 
blood,  is  then  brought  back  to  the  heart,  entering  its  right  auricle. 

2d.  The  Pulmonary. — The  venous  blood,  thus  brought  to  the  right  auricle,  is  forced 
into  the  right  ventricle  when  the  auricle  contracts.  A  similar  contraction  of  the  right 
ventricle  now  forces  it  into  the  pulmonary  arteries,  by  which  it  is  distributed  upon  the 
air-cells  of  the  lungs ;  here  a  chemical  change  takes  place  ;  from  being  dark  venous 
blood,  it  becomes  bright  red  arterial,  and,  being  collected  by  the  pulmonary  veins,  re- 
turns, finally,  to  the  left  auricle  of  the  heart.  In  the  pulmonary  circulation,  therefore, 
the  blood  leaves  the  right  ventricle  for  the  lungs,  undergoes  in  them  a  change,  giving 
off  carbonic  acid,  and  then  returns  to  the  left  auricle. 

3d.  The  Portal. — The  portal  vein  collecting  blood  from  the  chylopoietic  viscera, 
distributes  it  to  the  liver.  This  blood  there  mingles  with  that  which  has  been  derived 
from  the  hepatic  artery,  and  which  has  already  been  deoxydized  in  that  organ.  From 
this  portal  blood,  bile  is  secreted  and  passed  into  the  biliary  tubes.  The  changed  blood 
is  now  collected  by  the  ramifications  of  the  hepatic  veins,  along  which  it  is  transmitted 
to  the  ascending  vena  cava.  The  portal  circulation  is,  therefore,  apparently  not  con- 
nected with  any  central  organ  of  impulse.  It  commences  in  a  capillary  system,  and 
terminates  in  a  capillary  system.  There  is  no  hydraulic  mechanism  for  the  purpose 
of  determining  a  current. 

107.  Of  these  different  varieties  of  circulation,  let  us  now  select  one,  and  show  that 
the  principles  we  have  employed  in  describing  the  flow  of  sap  both  upward  and  down- 
ward, apply  here  also.    Let  us,  therefore,  confine  for  the  present  our  considerations  to 


/ 


CAPILLARY  RELATIONS  OF  ARTERIAL  AND  VENOUS  BLOOD  TO  THE  TISSUES.  35 

the  systemic  circulation,  and  tiien  proceed  to  show  how  the  same  principles  hear  on 
the  other  varieties  also.  For,  as  has  already  been  remarked,  it  is  essential  for  a  true 
theory  of  these  movements,  not  only  to  explain  them  in  an  isolated  case,  l)ut  also  in 
every  instance  ;  to  explain  not  only  the  systemic,  but  also  the  pulmonary  and  the  portal; 
to  explain  not  only  the  varieties  of  circulatory  movement  in  one  given  individual,  but 
also  in  every  tribe,  no  matter  whether  they  be  low  or  higb  in  the  scale  of  creation,  or 
whether  they  belong  to  the  animal  or  vegetable  world. 

108.  Let  us,  therefore,  take  as  our  guide  the  great  principle  laid  down  in  the  pre- 
ceding chapter  (84-88),  That  if  two  liquids  communicate  with  one  another  in  a  caiiillary 
tube,  or  in  a  porous  or  parenchymatous  structure,  and  have  for  that  tube  or  structure  dif- 
ferent chemical  affinities,  movement  will  ensue  ;  that  liquid  ivhich  has  the  most  energetic 
affinitij  will  move  ivith  the  greatest  velocity,  and  may  even  drive  the  other  liquid  entirely 
before  it. 

109.  The  arterial  blood  which  moves  along  the  various  aortic  branches,  and  is  dis- 
tributed to  every  part  of  the  system,  contains  oxygen,  which  it  has  derived  during 
its  passage  through  the  lungs.  Its  colour  is  crimson.  As  soon  as  it  has  reached  its 
destination  in  the  minute  capillary  vessels,  it  begins  to  carry  on  its  proper  process  of 
oxydation,  attacking  in  a  measured  way  the  various  tissues  through  which  it  is  flowing, 
burning  out  their  effete  carbonaceous  matter,  perhaps  also  burning  their  hydrogen  into 
water.  The  direct  result  of  this  operation  is  an  evolution  of  heat.  But  while  this 
chemical  change  in  the  tissues  is  going  forward,  the  arterial  blood  itself  is  also  suffering 
a  change  in  giving  up  its  oxygen,  which  may  be  looked  upon  as  its  active  principle, 
and  gaining  in  exchange  the  results  of  the  combustion.  From  being  crimson,  it  turns 
dark ;  from  being  arterial,  it  changes  into  venous  blood. 

110.  Let  us  farther  confine  our  thoughts  to  what  nujst  take  place  in  a  single  cap- 
illary tube,  or  in  one  small  portion  of  a  porous  structure;  for  whatever  reasoning  holds 
in  this  case,  will  also  hold  for  any  number  of  capillary  tubes,  or  any  mass  of  parenchy- 
matous structure.  On  the  arterial  side  of  such  a  tube  we  have  the  crimson  arterial 
blood;  on  the  venous  side  we  have  the  dark  venous  blood — two  different  fluids;  but 
what  is  the  relation  which  obtains  between  each  of  these  liquids,  and  the  walls  of  the 
tube  or  the  substance  of  the  parenchyma  in  which  they  are  placed  ?  Must  it  not  be 
that  the  arterial  blood,  bearing  its  oxygen,  ready  to  burn  out  any  carbon  or  hydrogen 
in  its  way,  substances  of  which  the  tube  or  structure  is  composed,  possesses  an  intense 
affinity  for  those  structures,  an  affinity  which  is  at  last  exhil)ited  by  its  actual  destruc- 
tion of  them  !  The  arterial  blood,  therefore,  has  an  intense  affinity  for  any  of  the 
tissues  with  which  it  is  brought  in  contact. 

111.  In  the  next  place,  how  is  it  with  the  venous  blood,  which  occupies  the  other 
extremity  of  the  tube  ?  The  affinities  of  the  oxygenized — the  arterial  blood — have  been 
satisfied  ;  it  has  effected  the  combustion  of  the  tissues  through  which  it  has  gone,  it  has 
changed  into  inert  venous  blood.  From  being  red,  it  has  turned  dark.  The  very 
change  which  has  come  upon  it,  or  which,  rather,  it  has  undergone,  is  sufficient  to  as- 
sure us  that,  so  far  as  its  chemical  affinities  for  the  surrounding  structures  are  concerned, 
those  affinities  are  at  an  end.  The  venous  blood,  therefore,  has  little  affinity  for  any 
of  those  structures  with  which  it  is  brought  in  contact. 


36 


CAUSE  OF  THE  PULMONARY  CIRCULATION. 


112.  To  the  mind  of  a  chemist,  the  relation  which  exists  between  arterial  and  venons 
blood  and  the  soft  solids  of  the  animal  body  may  be  very  forcibly  impressed  by  these 
considerations.  Regarding  the  blood  itself  as  a  mere  vehicle  for  the  introduction  of 
oxygen,  and  the  carrying  away  of  carbonic  acid  and  water,  and  remembering  that  the 
substances  of  the  tissues  acted  upon  are  chiefly  carbon  and  hydrogen,  our  final  estimate 
of  the  relation  of  arterial  and  venous  blood  respectively  to  those  tissues  comes  to  this — 
the  affinity  of  arterial  blood  is  expressed  by  the  affinity  of  oxygen  for  carbon  and  hy- 
drogen ;  the  affinity  of  venous  blood  by  that  of  carbonic  acid  for  carbon,  and  of  water 
for  hydrogen.  Compared  together,  therefore,  the  former  is  the  representative  of  a 
highly  energetic  force,  which  in  the  latter  is  diminished  down  to  zero. 

113.  Now  what  is  the  phenomena  which  our  general  principle  (108)  predicts,  as 
arising  under  these  circumstances  1  Simply  this,  that  the  arterial  will  drive  the  venous 
blood  before  it,  and  drive  it  with  an  inexpressible  force. 

114.  The  oxygenizing  action  of  the  arterial  blood  is,  therefore,  the  true  cause  of  the 
systemic  circulation. 

115.  In  the  systemic  circulation,  upon  these  principles,  the  flow  must  be  from  the 
artery  to  the  vein. 

116.  The  pulmonary  circulation  next  presents  itself  In  this,  as  we  have  described 
(106),  there  is  a  certain  chemical  change  going  on,  the  consideration  of  which  gives 
us  the  cause  of  the  movement.  We  have  seen  that  the  motive  force  of  the  systemic 
circulation  arises  from  the  deoxydation  of  arterial  blood  (114).  How  is  it  with  the 
pulmonary  1  We  have  here  venous  blood  presenting  itself  on  the  air-cells,  no  longer 
presenting  itself  to  carbonaceous  or  hydrogenous  atoms,  such  as  constitute  the  soft 
solids,  but  presenting  itself  to  atmospheric  air,  or,  more  truly,  to  oxygen  gas  itself,  which, 
being  the  more  absorbable  of  the  constituents  of  the  air,  is  taken  up  and  held  in  solu- 
tion by  the  moist  walls  of  the  air-cells.  Under  these  circumstances,  we  see  plainly 
that  we  are  considering  a  case  which  is  precisely  the  converse  of  the  former;  in  that 
the  arterial  blood  had  an  intense  affinity  for  the  carbonaceous  substances  with  which 
it  was  brought  in  contact,  and  the  venous  none.  In  this,  the  venous  blood  has  a  cor- 
responding intense  affinity  for  the  oxygen  which  is  dissolved  in  the  tissues  with  wliich 
it  is  in  contact,  and  the  arterial  blood  has  none.  Movement  again  must  ensue,  but  as 
the  conditions  of  the  affinity  are  reversed,  so  also  is  the  direction  of  the  motion,  for 
now  the  venous  blood  drives  the  arterial  before  it,  and  drives  it  with  an  inexpressible 
force  to  the  heart. 

117.  The  pulmonary  circulation  is,  therefore,  due  to  the  oxydation  of  the  venous 
blood. 

118.  The  direction  of  the  pulmonary  circulation  ought  to  be  from  the  venous  to  the 
arterial  side. 

119.  Had  we,  therefore,  known  nothing  of  the  circulation  in  the  higher  order  of 
animals,  but  been  instructed  in  the  chemical  relations  of  the  blood  to  the  soft  tissues 
anil  atmospheric  air,  we  could,  upon  physical  principles,  have  predicted  the  existence 
of  that  circulation,  and  shown  what  its  direction  in  different  organs  must  be. 

120.  It  may  strike  those  who  are  not  familiar  with  physiological  facts,  that  in  these 


USES  AND  ACTION  OF  THE  HEART.  37 

descriptions  a  most  important  omission  is  made — the  omission  of  tlie  action  of  the 
heart,  an  organ  plainly  connected  by  position  and  mechanism  with  the  phenomena  we 
have  under  consideration. 

121.  Physiologists  have  long  seen,  in  opposition  to  the  popular  opinion,  that  the 
heart  can  only  exert  a  very  subsidiary  action.  Plants  are  wholly  destitute  of  such  an 
organ,  yet  their  juices  circulate,  and  there  are  multitudes  of  animals  which  are  in  the 
same  predicament.  In  insects,  for  example,  for  reasons  for  which  we  can  give  on  these 
principles  a  clear  explanation,  no  such  central  organ  of  impulse  appears.  In  fishes,  the 
systemic  circulation  is  carried  on  without  a  heart,  and  in  cold-blooded  animals  move- 
ment in  the  capillaries  takes  place  after  the  heart  has  been  cut  out.  Even  in  man,  after 
death,  the  arterial  tubes  are  found  for  the  most  part  empty,  and  it  is  inconceivable  that 
this  should  have  happened  through  any  possible  agency  of  tbe  heart  itself,  but  meets 
with  a  very  ready  explanation  upon  the  principles  we  have  been  giving.  In  the  case  of 
acardiac  monsters,  also,  which  are  not  uncommon,  the  absence  of  a  heart  seems  to  have 
exerted  little  agency  on  development  and  growth.  Moreover,  when  we  inquire  into 
the  condition  of  the  circulation  in  the  earliest  periods  of  existence,  we  find  that,  far 
from  the  heart  being  the  first  to  appear  as  a  central  point,  and  its  various  vessels  to  branch 
forth  from  it,  the  vessels  themselves  are  the  first  to  appear,  and  the  heart,  then,  is  subse- 
quently developed. 

122.  What,  then,  is  the  true  office  of  the  heart  1  How  is  it  that  it  comes  to  form 
so  essential  a  portion  of  this  mechanism  1  Had  the  Creator  predetermined  that  in 
the  construction  of  tbe  circulatory  apparatus  of  man  no  organ  of  the  kind  should  be 
employed,  let  us  consider  the  final  difficulties  which  must  have  arisen.  The  systemic 
circulation  originates  in  the  deoxydation  of  the  blood,  the  pulmonary  in  its  oxydation. 
We  can  conceive  that  advantage  might  have  been  taken  of  the  excess  of  mechanical 
force  arising  at  the  points  where  these  chemical  changes  were  going  on,  for  that  model 
was  successfully  followed  in  the  systemic  circulation  of  fishes  ;  but  in  the  case  of  ani- 
mals which  do  not  live  in  the  sea,  but  breathe  atmospheric  air,  there  are  serious  diffi- 
culties in  the  way.  It  is  very  apparent  that  there  is  no  necessary  connexion  between 
the  c4ieniical  changes  taking  place  in  the  lungs  and  those  taking  place  in  the  system. 
The  rate  of  oxydation  in  the  lungs  depends  on  a  variety  of  causes,  and  the  rate  of  de- 
oxydation is  also  determined  by  its  own  proper  causes.  There  is  no  necessary  con- 
nexion between  them  ;  and  if,  on  either  of  these  points  of  change,  an  excess  of  action 
took  place,  the  result  of  it  must  have  been  a  disturbance  of  the  equilibrium  of  the  whole 
circulation.  At  some  central  point,  therefore,  the  current  going  to  the  respiratory  ma- 
chine and  that  going  to  the  system  must  be  intercepted — and  intercepted  by  an  appa- 
ratus which  could  hold  both  in  check,  and  time  the  movements  of  the  one  to  the  move- 
ments of  the  other.  In  animal  structures,  motions  of  this  kind  are  universally  accom- 
plished by  the  help  of  muscular  contractions,  and  hence  arose  the  idea  of  a  heart,  which, 
by  periodic  muscular  contractions,  should  serve  to  adjust  the  flowing  currents  to  one 
another,  and  prevent  engorgements  or  deficiencies  in  any  part  of  the  route  taking  place. 

123.  From  this  arrangement,  also,  another  important  advantage  arose.  The  arterial 
and  venous  tubes  in  the  neighbourhood  of  the  heart  attain  a  considerable  diameter. 


38 


CAUSE  OF  THE  PORTAL  CIRCULATION. 


From  this  circumstance,  they  require  some  arrangement  by  which  they  can  be  filled  or 
emptied.  The  auricles  of  the  heart  relieve  all  their  corresponding  veins,  to  a  very 
great  extent,  from  pressure ;  the  ventricles  serve,  in  their  turn,  to  fill  thoroughly  the 
aorta  and  the  pulmonary  artery. 

124.  In  the  last  place,  let  us  turn  to  the  portal  circulation,  and  show  how,  in  it,  the 
same  physical  principles  apply.  The  blood  which  flows  tovi^ards  the  liver,  along  the 
portal  vein,  has  been  obtained  by  that  vein  from  the  chylopoietic  viscera ;  it  has,  there- 
fore, the  same  relation  to  the  blood  furnished  from  the  different  and  corresponding  aor- 
tic branches  as  has  the  general  systemic  venous  blood.  The  arterial  blood,  therefore, 
drives  it  before  it,  in  the  same  way  that  the  general  systemic  circulation  takes  place, 
and,  passing  along  the  portal  vein,  it  is  now  distributed  to  the  liver.  In  this  organ  it 
also  receives  the  blood  which  has  been  brought  by  the  hepatic  artery,  and  which  has 
served  lor  the  purposes  of  the  liver. 

125.  The  process  of  biliary  secretion  now  takes  place,  and  compounds  of  carbon 
and  hydrogen  along  with  soda  are  separated  as  bile,  and  pass  along  the  biliary  tubes. 
In  its  final  effect,  therefore,  the  chemical  action  of  the  liver  closely  resembles  the  chem- 
ical action  of  the  lungs.  Compared  with  the  resulting  blood  which  passes  along  the 
branches  of  the  hepatic  veins,  and  finds  its  way  into  the  ascending  vena  cava,  the  por- 
tal blood  differs  by  containing  the  elements  of  bile. 

126.  Two  systems  of  forces  now  conspire  to  drive  the  portal  blood  out  of  the  liver 
into  the  ascending  cava.    Let  us  consider  them  in  succession. 

1st.  The  blood  which  is  coming  along  the  capillary  portal  veins,  and  that  which  is 
receding  by  the  hepatic  veins,  compared  together,  as  to  their  affinities  for  the  structure 
of  the  hver,  obviously  have  this  relation  ;  the  portal  blood  is  acted  upon  by  the  hver, 
and  there  is  separated  from  it  the  constituents  of  the  bile ;  the  affinities  which  have 
been  at  work  in  producing  this  result  have  all  been  satisfied,  and  the  residual  blood, 
over  which  the  liver  can  exert  no  action,  constitutes  that  which  passes  into  the  hepatic 
veins.  Between  the  portal  blood  and  the  structure  of  the  liver  there  is  an  energetic 
affinity,  betrayed  by  the  circumstance  that  a  chemical  decomposition  takes  place,  and 
bile  is  separated ;  and  that  change  completed,  the  residue,  which  is  no  longer  acted 
on,  forms  the  venous  blood  of  the  hepatic  veins.  In  the  same  manner,  therefore,  that 
in  the  systemic  circulation  arterial  blood,  in  its  passage  along  the  capillaries,  becomes 
deoxydized  in  consequence  of  an  affinity  between  its  elements  and  those  of  the  struc- 
tures with  which  it  is  brought  in  contact,  and  drives  the  inert  venous  blood  before  it, 
so,  too,  in  the  portal  circulation,  in  consequence  of  the  chemical  affinities  and  reactions 
which  obtain  between  the  portal  blood  and  the  substance  of  the  liver — affinities  and 
reactions  which  are  expressed  bv  the  separation  of  the  bile — that  blood  drives  before  it 
the  inert  blood  which  is  found  in  the  hepatic  veins. 

2d.  But  in  the  liver  there  is  a  second  agency  at  work,  which,  conspiring  in  its  re- 
sultant with  the  former,  produces  motion  in  the  same  direction.  As  we  have  said,  the 
blood  of  the  .'hepatic  artery,  after  serving  for  the  economic  purposes  of  the  liver,  is 
thrown  into  the  portal  plexus.  Hence  arises  a  second  force.  The  pressure  of  the  ar- 
terial blood  in  the  hepatic  capillaries  upon  this,  is  sufficient,  not  only  to  impel  it  info 


THREE  SOURCES  OF  FORCE  IN  THE  PORTAL  CIRCULATION.  39 

the  capillaries  of  the  portal  veins,  but  also  to  give  it  a  pressure  in  a  direction  towards 
the  hepatic  veins.  For  any  pressure  which  arises  between  the  arterial  blood  of  the 
hepatic  and  its  corresponding  venous  blood,  must  give  rise  to  motion  towards  the  he- 
patic veins ;  no  regurgitation  can  take  place  backward  through  the  portal  vein  upon  the 
■  blood  arriving  from  the  chylopoietic  viscera,  because  along  that  channel  there  is  a  pres- 
sure  propagated  in  the  opposite  direction,  arising  from  the  arterial  blood  of  the  aortic 
branches.  The  pressure,  therefore,  arising  from  the  relations  of  the  hepatic  arterial 
blood,  conspires  with  that  arising  from  the  pressure  of  the  portal  blood,  and  both  to- 
gether join  in  giving  rise  to  motion  towards  the  ascending  cava. 

127.  So  great  are  the  forces  which  arise  under  these  circumstances,  that  there  is  no 
doubt  that  the  blood  of  the  hepatic  artery  alone  could,  of  itself,  control  the  circulation 
of  the  liver  without  any  re-enforcing  aid  from  the  portal  blood.  So  when,  from  acci- 
dent or  otherwise,  the  portal  vein  is  shut,  or  when  from  malformation  it  opens  directly 
into  the  ascending  cava,  the  hepatic  artery  takes  charge  of  the  functions  of  the  liver, 
and  directs  its  circulatory  conditions. 

128.  These  views,  therefore,  lead  us  to  understand  that  there  are  three  sources  of 
force  engaged,  under  normal  circumstances,  in  directing  the  portal  circulation.  One 
of  these  is  found  in  the  aortic  capillaries,  when  they  are  spread  on  the  chylopoietic 
viscera,  the  mode  of  action  being  precisely  analogous  to  that  which  obtains  in  the  gen- 
eral systemic  circulation.  The  other  two  are  found  in  the  liver  itself;  the  first  is  a 
pressure  exerted  by  the  portal  blood  on  that  of  the  hepatic  veinlets ;  the  second  by  the 
blood  of  the  hepatic  artery,  which,  conspiring  with  the  former,  urges  the  resulting  mix- 
ture along  the  hepatic  veins  into  the  ascending  cava. 

129.  I  might  now  proceed  to  show  with  what  clearness  these  doctrines  explain  the 
circulation  of  the  blood  in  other  tribes  of  life ;  for  example,  in  the  case  of  the  model 
which  is  adopted  in  fishes,  the  aorta  of  which  has  long  ago  been  recognised  as  bearing 
a  strong  resemblance  to  the  portal  vein  of  the  mammalia ;  but,  as  throughout  this  chap- 
ter these  latter  have  been  constantly  referred  to,  I  shall  continue  what  is  here  to  be  of- 
fered farther  by  using  their  type  of  construction  for  illustration.  To  any  one  who  re- 
flects on  the  principles  which  have  been  laid  dowai,  there  w  ill  arise  no  difficulty  in  ex- 
plaining the  mechanical  causes  of  the  circulation  in  any  particular  case,  more  especially 
if  this  plain  precept  is  kept  constantly  in  mind,  that,  for  the  physical  reasons  luhich  have 
been  assigned  (88),  a  pressure  will  always  he  exerted  in  every  one  of  these  instances  hy  the 
Jluid  which  is  ready  to  undergo  a  change  upon  that  which  has  already  undergone  it;  a 
pressure  which,  as  there  is  no  force  to  resist  it,  will  always  give  rise  to  motion  in  a  direc- 
tion from  the  changing  to  the  changed  fluid. 

130.  As  we  have  said,  it  is  the  character  of  a  true  theory  to  be  applicable  to  all 
cases,  and  to  render  a  clear  account  of  every  circumstance  that  may  arise.  A  true  the- 
ory is  like  a  window  of  crystal  glass,  through  which  we  can  see  all  objects  in  their 
proper  positions,  and  colours,  and  relations,  no  matter  whether  they  are  such  as  are 
near,  or  those  that  are  at  a  distance  ;  no  matter  whether  they  are  directly  before  us,  or 
enter  only  obliquely  into  the  field  of  view.  A  fictitious  theory  is  like  a  Venitian  blind, 
which  has  to  be  set  in  a  certain  position  with  respect  to  the  observer,  and  only  shows 


40 


ACTION  IN  ASPHYXIA,  ETC. 


him  objects  for  which  it  has  been  adjusted,  and  those  in  an  unsatisfactory  manner ;  but 
if  he  moves  to  one  side  or  to  the  other,  or  endeavours  to  see  objects  which  are  not  di- 
rectly in  his  way,  his  view  is  intercepted,  or,  perhaps,  unless  he  makes  a  new  adjust- 
ment, tiie  light  is  shut  out  altogether. 

131.  Let  us,  therefore,  see  how  these  chemical  principles  will  apply  when  the  sys- 
tem under  consideration  is  no  longer  under  normal  conditions,  but  has  passed  into  a 
state  of  disturbance  or  of  disease.  It  is  well  known  to  physicians,  from  the  phenomena 
of  asphyxia,  thai  whenever  the  admission  of  oxygen  into  the  air-cells  of  the  lungs  is 
prevented,  the  circulation  through  them  simultaneously  stops ;  but  it  may  be  renewed 
again  on  the  readmission  of  that  gas,  provided  it  is  within  a  short  space  of  time,  and 
recovery  may  and  often  does  take  place  under  these  circumstances.  What,  now,  is  the 
cause  of  that  asphyxiated  condition  1  why  does  the  blood  cease  to  flow  1  The  chem- 
ical theory  of  the  circulation  of  the  blood  through  the  lungs,  which  I  have  just  given, 
points  to  the  oxydation  of  that  blood  as  the  very  cause  of  its  movement  (116).  It  is 
the  pressure  of  the  deoxydized  upon  the  oxydized  blood  that  drives  the  latter  along  the 
pulmonary  veins  to  the  heart.  But  should  anything  intervene  to  prevent  that  oxyda- 
tion taking  place,  no  pressure  can  arise,  and,  therefore,  no  movement  can  ensue ;  the 
conditions  for  asphyxia  are  all  present;  conditions  which,  however,  are  removed  so 
soon  as  oxydation  can  be  reaccomplished ;  then  movement  once  more  takes  place,  and 
a  natural  state  is  restored.  If  any  evidence  were  required  to  show  how  litile  influence 
the  heart  possesses  in  controlling  the  pulmonary  circulation,  and  how  much  depends  on 
the  oxydation  of  the  blood,  it  may  be  derived  from  the  phenomena  of  asphyxia. 

132.  As  another  illustration,  we  may  here  bring  forward  and  explain  the  fact,  well 
known  to  physiologists,  that  if  the  trachea  be  obstructed  so  that  oxygen  is  restrained 
from  passing  into  the  lungs,  and  asphyxia  is  being  induced,  blood  taken  from  any  of 
the  systemic  arteries  exhibits  a  venous  aspect.  What,  now,  does  the  chemical  theory 
say  should  take  place  in  the  general  circulation]  We  are  to  remember  (114)  that  the 
very  cause  of  that  circulation  arises  from  the  pressure  of  the  oxydized  arterial  blood 
upon  the  deoxydized  venous  blood.  Under  the  conditions  supposed,  we  have  inter- 
fered with  the  constitution  of  the  arterial  blood,  and  given  it  a  venous  character;  the 
conditions  for  pressure  are,  therefore,  not  accomplished,  and  no  pressure  takes  place, 
and  no  flow  from  the  arterial  to  the  venous  capillaries.  Under  these  circumstances,  the 
impulsive  action  of  the  left  ventricle  must  be  spent  in  an  increased  pressure  on  the 
walls  of  all  its  communicating  arteries,  and  a  simultaneous  relief  of  pressure  takes  eflect 
on  the  walls  of  all  the  veins.  As,  in  a  steam-engine,  any  steady  variation  of  motion  is 
finally  impressed  upon  the  governor,  which  adjusts  itself  consentaneously;  so,  in  this 
case,  the  heart,  which  acts  as  a  governor,  accommodates  itself  to  the  changes  going  on. 
The  left  ventricle  presently  ceases  in  its  violent  effort  to  force  the  blood,  and  the  pres- 
sure on  the  arterial  walls  abates.  But,  if  the  trachea  is  now  relieved,  oxydation  of  the 
blood  goes  on,  pressure  again  takes  place  in  the  systemic  capillaries,  and  the  proper 
action  is  restored. 

133.  These  different  facts,  which  might  have  been  predicted  from  the  theory  that 
the  deoxydation  of  arterial  blood  is  the  cause  of  the  systemic  (-irculation.  have  every 


ON  THE  PHYSICAL  CONSTITUTION  OF  THE  SUNBEAM.  41 

one  been  experimentally  determined  by  direct  experiment  by  Dr.  Riid  {Carpenter s 
Human  Phys.,  p.  417). 

134.  We  mi£:ht  now  proceed  to  inquire  what,  on  these  chemical  principles,  ouglit  to 
take  place  when  a  reverse  action  ensues,  as  when  between  tlie  arterial  blood  and  the 
tissues  through  which  it  flows  those  conditions  are  set  up  which  lead  to  its  more  rapid 
deoxydation,  and  the  concomitant  evolution  of  heat ;  conditions  which  are  found  in 
the  various  local  inflammations.  In  these,  by  reason  of  an  increased  affinity  between 
the  soft  tissues  and  the  arterial  blood,  deoxydation  and  corresponding  combustion  more 
rapidly  take  place,  with  an  abnormal  elevation  of  temperature,  and  the  flow  of  blood 
to  the  point  of  disturbance  is  increased.  The  old  medical  aphorism,  ''uhi  irritatio  ihi 
affluxus,"  translated  into  the  precise  language  of  modern  chemistry,  simply  means,  to 
the  iioirit  where  its  deoxydation  is  taking  place,  the  arterial  blood  will  jlow. 

135.  By  the  aid  of  the  principles  here  laid  down,  all  the  various  physiological  or 
pathological  conditions  which  are  met  with  in  inflammation,  ;  sphyxia,  gangrene,  &c., 
are  presented  as  so  many  interesting  chemical  problems  for  solution.  Such  cases,  also, 
as  the  non-asphyxiation  of  reptiles,  the  variable  respiration  and  heat  of  insects,  accord- 
ing as  they  are  in  motion  or  at  rest,  the  results  of  death  from  lightning,  aflord  abundant 
opportunity  for  tlie  full  verification  of  these  doctrines.  That  the  time  has  now  ar- 
rived when  the  exact  sciences  are  to  come  to  the  aid  of  physiology,  no  one  can  doubt. 
For  many  centuries  past,  a  profitless  system  has  been  followed,  the  same  system  that 
formerly  obtained  in  natural  philosophy,  and  the  uncertainties  and  doubts  of  medical 
science  are  the  best  proofs  of  its  value.  It  is  the  ruling  principle  of  this  system  to  sat- 
isfy the  inquirer  for  facts  by  the  use  of  empty  words,  words  which  mean  nothing  and 
prove  nothing.  Life  and  vitahty,  with  other  sonorous  epithets,  figure  away  in  these 
visionary  speculations  as  though  they  were  realities,  and  change  their  forms  without 
reason  or  rule,  as  do  the  images  that  we  see  in  dreams. 


CHAPTER  IV. 

ON  THE  PHYSICAL  CONSTITUTION  OF  THE  SUNBEAMS  AND  ON  THE  PRISMATIC  SPECTRUM. 

Contents  :  Modes  of  isolating  the  Coloured  Rays. — Newton  s  Prismatic  Spectrum. — 
Theory  of  the  Colours  of  Light. — Illuminating  Calorific  and  Chemical  Po^vers  of  the 
Spectrum. — Newton  s  Processes for  purifying  the  Spectrum. — Fixed  Lines. — Melloni^s 
Experiments  on  the  Distribution  of  Heat. — Physical  Independence  of  Heat. — HerscheTs 
Experiments  on  the  Thermic  Spectrum. — Chemical  Action  of  the  different  Regions 
of  the  Spectrum  on  a  Daguej-reotype  Plate. — Chemical  Action  on  other  Bodies. 

136.  From  the  foregoing  observations  (Ch.  I.),  we  see  that  the  primary  formation 
of  organized  from  inorganic  matter  is  brought  about  by  the  agency  of  the  sunbeam, 
either  directly  falling  on  the  point  of  change,  or  received  in  an  indirect  way,  as  the 
diff"nsed  light  of  the  sky  or  clouds  (56). 

F 


42 


VARIOUS  MODES  BY  WHICH  COLOUR  IS  PRODUCED. 


137.  Ill  the  sunbeam  dififerent  principles  exist,  some  of  wliich  are  visible  to  the  eye 
and  others  invisil)Ie.  The  conjoint  action  of  the  former  communicates  to  us  an  im- 
pression of  what  we  denominate  white  light.  But  in  this  white  light  there  are  rays 
endowed  with  the  quality  of  exciting  the  sensations  of  colour,  such  as  red,  yellow, 
green,  blue.  We  are  required  to  determine  which  of  these  principles  is  concerned  in 
the  physiological  change  we  are  considering.    It  is  necessary,  therefore,  to  describe 

the  SOLAR  SPECTRUM. 

138.  A  beam  of  light  coming  into  a  room  through  an  ordinary  window  is  percepti- 
ble from  all  parts,  for  dust  or  other  heterogeneous  particles  which  are  always  floating 
in  the  air  scatter  the  rays  in  all  directions  by  reflexion,  and  enable  them  to  operate  on 
the  eye.  This  light  constitutes  what  is  designated  by  optical  writers  as  white  light. 
To  the  ordinary  acceptation  of  the  term  white,  it  does  not  correspond  ;  it  is  the  un- 
changed, unaffected  light  of  the  sun. 

139.  A  beam  of  light  coming  through  the  painted  window  of  a  cathedral  falls  on 
the  ground,  or  on  objects  in  its  way,  and  communicates  to  them  the  various  tints  with 
which  the  glass  has  l)een  stained.  The  brilliant  colours  which  are  thus  developed  by 
the  action  of  the  glass  exist  originally  in  the  white  light,  and  are  made  apparent  by 
the  absorptive  action  of  the  medium  through  which  they  have  come,  in  a  way  which  will 
be  presently  explained. 

140.  There  are  also  other  modes  by  which,  from  white  light,  brilliant  colours  can  be 
produced.  Thus,  transparent  media,  such  as  gems,  cut  into  certain  shapes  and  polished, 
when  exposed  to  the  light  glisten  with  a  play  of  colour.  It  is  these  brilliant  hues  which 
give  to  the  diamond  its  value  as  an  article  of  female  ornament.  It  is  also  in  the  same 
way  that  the  angular  pieces  of  glass  which  are  strung  upon  chandeliers  and  around  gas 
flames,  emit  in  a  brilliantly-lighted  room  so  many  fitful  changes  of  colour.  And  in  the 
same  way  also  Nature  exhil)its  to  us  that  most  beautiful  of  all  meteorological  phenomena, 
the  rainl)ow,  by  refracting,  reflecting,  and  dispersing  the  white  light  of  the  sun,  and  pro- 
ducing a  regular  display  of  colours.  These  colours,  arranged  as  in  the  rainbow,  may 
also  be  seen  wherever  a  shower  of  drops  of  water  is  falling  in  a  proper  position  as  re- 
spects the  spectator  and  the  sun  ;  they  are  often,  therefore,  visible  w  hen  fountains  are 
playing;  often  in  the  sea-spray,  when  it  is  cast  ashore  by  a  brisk  wind;  often  accom- 
panying the  bows  of  a  steamboat  which  is  moving  rapidly  through  the  water. 

141.  There  is  a  third  mode  by  which,  from  white  light,  brilliant  colours  are  produ- 
ced. It  is  by  the  interference  of  rays ;  in  the  same  manner  that  two  sounds  may  be 
so  situated  with  respect  to  each  other  as  to  destroy  one  another's  eflect,  and  produce 
silence;  or,  as  two  waves  upon  water,  when  the  concavity  of  the  one  corresponds  with 
the  convexity  of  the  other,  destroy  one  another's  effect ;  so  may  two  rays  or  waves 
of  light  be  placed,  in  respect  of  each  other,  that,  instead  of  re-enforcing  each  other's  ef- 
fect, they  may  produce  darkness.  This  phenomenon,  which  in  these  different  cases 
passes  under  the  general  designation  of  interference,  under  ordinary  circumstances,  in 
the  case  of  light,  is  expressed  by  the  production  of  brilliant  colours.  Such  are  the 
beautiful  and  almost  metallic  tints  that  are  seen  on  the  wing-cases  of  certain  coleopte- 
rous insects,  more  especially  certain  beetles  in  the  Southern  States,  which  expose  a 


NEWTON'S  PRISMATIC  SPECTRUM.  43 

glow  of  colours — greens,  yellows,  blues,  and  reds — as  they  change  their  position.  So, 
too,  the  beautiful  display  which  is  seen  on  the  breast  of  the  wild  dove,  and  still  more 
beautifully  in  the  feathers  of  the  humming-bird. 

142.  The  cause  of  these  colours,  and  of  the  colours  of  light  generally,  was  first  dis- 
covered by  Sir  Isaac  Newton,  who,  in  his  investigations,  followed  the  second  of  the 
methods  here  mentioned,  the  production  of  colour  by  refraction.  Of  late  years,  the  at- 
tention of  philosophers  has  been  much  turned  to  the  third  method,  that  of  interference  ; 
this,  as  will  be  presently  seen,  possesses,  in  particular  cases,  very  great  advantages  over 
that  by  refraction  (140),  or  the  first  one,  by  absorption  (139). 

143.  Newton's  methods  of  investigating  the  production  of  colour  by  refraction  may 
be  briefly  described,  as  follows:  Let  a  beam  of  light  coming  from  the  sun  {s,Jig.  Ill) 
pass  tlirough  a  circular  hole  (a  b)  in  the  shutter  of  a  dark  room,  and  fall  upon  a  piece 
of  glass  cut  into  the  form  of  a  triangular  prism,  c  d  e,  placed  as  in  the  figure.  On  the 
farther  side  of  the  prism  let  there  be  a  sheet  of  white  card-board,  M  N,  or  some  other 
such  screen,  to  receive  the  rays.  The  light,  on  its  entrance  into  the  room  through  the 
hole  in  the  shutter,  is  white,  and  were  the  prism  not  interposed,  it  would  advance  for- 
ward on  a  straight  line,  and,  falling  on  the  screen  at  z,  would  there  give  a  circular 
white  image  of  the  sun.  The  interposition  of  the  prism  disturbs  this  white  light  from 
its  rectilinear  course,  and,  refracting  it,  bends  it  into  a  new  path.  The  disturbance  pro- 
duced by  the  prism  is  of  two  kinds :  first,  the  ray  is  moved  out  of  the  rectilinear  posi- 
tion in  which  it  would  have  gone  ;  and,  second,  it  no  longer  gives  rise  to  a  white  circu- 
lar image,  but  to  an  elongated  and  highly-coloured  image,  which  goes  under  the  name 
of  the  solar  spectrum.  Of  the  intensity  and  beauty  of  these  colours  it  is  impossible  to 
give  any  description  by  words.  An  imperfect  representation  of  their  position  is  given 
in  the  frontispiece.  In  the  coloured  figure  thus  received  on  the  card-board,  Newton  de- 
tected seven  different  tints,  red,  orange,  yellow,  green,  blue,  indigo,  violet.  Of  these 
colours,  the  red  is  uniformly  nearest  to  the  point  {z)  to  which  the  ray  would  have  gone 
had  the  prism  not  intervened  ;  the  violet  is  most  distant.  And  as  the  production  of 
the  colours  depends  upon  the  refracting  action  of  the  prism,  Newton  designated  the 
red  as  the  least  refrangible,  because  it  was  least  removed  from  its  natural  course  ;  and 
the  violet  as  the  most  refrangible,  because  it  was  the  most  removed. 

144.  Now  if  these  seven  coloured  rays  be  collected  together  again  by  any  appropri- 
ate arrangement,  so  as  all  to  fall  on  one  common  point,  that  point  will  be  of  a  brilliant 
white,  as  was  the  point  z.  If  two  of  them,  as  the  yellow  and  blue,  in  like  manner  be 
directed  together,  they  will  give  rise  to  a  green ;  or  the  red  and  blue  to  a  purple ;  or 
the  yellow  and  red  to  an  orange ;  showing,  therefore,  that  while  different  coloured  rays 
thus  give  rise  to  compound  tints,  all  the  rays  in  the  spectrum  converged  together  pro- 
duce white  light,  the  same  as  that  which  originally  came  through  the  aperture  a  h,  in 
the  shutter. 

145.  Newton,  therefore,  was  correct  in  the  explanation  which  be  gave  of  these  phe- 
nomena. That  the  white  light  of  the  sun  consists  of  seven  differently-coloured  rays, 
some  of  which  are  more,  and  some  less,  refrangible  by  glass.  That,  consequently,  when 
a  beam  falls  on  a  triangular  prism  of  glass,  its  constituent  rays  are  not  all  equally  re- 


ILLUMINATING  CALORIFIC  AND  CHEMICAL  POWER  OF  THE  SPECTRUM. 


fracted.  That  of  them  the  red  is  least,  and  the  violet  most  refracted,  and,  therefore, 
the  white  image  (2)  which  the  original  beam  would  have  given,  became  elongated  so 
as  to  form  the  solar  spectrum  in  which  the  constituent  colours  are  seen. 

146.  Leaving  now  the  optical  arrangement  by  which  this  phenomenon  is  produced, 
let  us  confine  our  considerations  to  the  spectrum  itself.  It  is  to  be  remarked  that  the 
purity  of  the  constituent  coloured  spaces  becomes  greater,  as  the  separation  of  the  rays 
is  more  perfect;  and,  as  is  shown  in  Jig.  Ill,  these  rays  are  diverging  from  one  an- 
other, it  is  obvious  that  the  farther  the  screen  on  which  they  fall  is  placed  from  the 
prism,  the  greater  will  be  the  apparent  dispersion  of  the  rays.  If  the  screen  is  brought 
close  to  the  prism,  the  colours  are  but  little  developed,  [)ecause,  not  being  yet  separated 
from  one  another,  their  mixture  produces  white  light ;  but  at  a  distance  of  twelve  or 
twenty  feet,  they  have  diverged  sufficiently,  and  each  one  appears  for  itself 

147.  Inspection  proves  that,  in  this  spectrum,  not  only  do  the  various  parts  differ  in 
respect  of  the  colours  they  show,  but  they  also  differ  in  their  intrinsic  brilliancy.  A 
piece  of  fine-printed  paper,  held  in  succession  in  the  different  colours,  is  legible  at  very 
different  distances.  Held  in  the  yellow,  it  may  easily  be  read  at  a  considerable  dis- 
tance ;  l)ut  held  in  the  violet,  it  must  be  seen  close  at  hand,  or  the  letters  cannot  be  dis- 
tinguished. The  otiier  coloured  spaces  possess  intermediate  powers,  and,  by  direct  ex- 
periments made  by  Fraunhofer,  it  appears  that  the  different  portions  of  the  Newtonian 
spectrum  have  their  order  of  ihumination,  as  is  expressed  in  the  following  table,  in 
which  it  will  be  perceived  that  the  brilliancy  of  the  yellow  rays  is  taken  as  unity,  and 
the  other  rays  compared  therewith.    The  letters  refer  to  the  figure  in  the  frontispiece. 

TABLE  OF  THE  ILLUMINATING  POWER  OF  THE  DIFFERENT  REGIONS  OF  THE  NEWTONIAN 

SPECTRUM. 


Iiileiisity  (il  Litjiit  1 

In  JB  C 
C  D 
n  E 

2  1 
29  9 
100  0 

In  E  F 
F  G 
G  H 

32  « 
18  5 
:  !  ■'  ! 

148.  It  is  not  only  in  illuminating  power  that  tliese  diflerent  regions  vary;  they  vary 
also  in  their  heating  power,  as  is  shown  by  their  action  on  a  thermometer.  When  a  prism 
made  of  flint  glass  is  used,  and  the  bulbs  of  a  set  of  small  and  delicate  thermometers 
are  plunged  in  the  coloured  spaces,  it  is  perceived  that,  commencing  with  the  violet, 
each  thermometer  rises  higher  as  it  approaches  the  red  region;  and  even  beyond  the 
red  region,  where  the  eye  can  detect  no  trace  of  light,  the  maximum  of  heat  occurs, 
thus  showing  that  the  heat  which  exists  in  the  sunbeam  is  an  intrinsically  different 
agent  from  the  light,  because,  by  the  action  of  a  prism,  it  can  be  refracted,  and  is  found 
in  a  space  in  which  no  light  exists. 

149.  It  has  been  known  for  a  long  time  that  the  white  chloride  of  silver,  and,  indeed, 
all  the  white  salts  of  silver,  when  exposed  to  the  sun,  turn  black  (Ap.,  443).  A  piece 
of  paper  washed  over  with  any  of  these  white  bodies,  as  the  chloride,  and  held  in  the 
specrrum,  soon  undergoes  a  change.  In  the  more  refrangible  region,  the  rays  begin  to 
effect  a  decomposition,  which  spreads  far  beyond  the  violet  extremity  ;  and  when  the 
bromide  of  silver  is  nsed,  this  darkening  action  is  simultaneously  begun  from  end  to  end 
of  the  spectrum,  and  chemical  action  extends  beyond  both  of  its  extremities.    In  the 


PROCESSES  FOR  PURIFYING  THE  SPECTRUM.  45 

same  manner,  therefore,  that  from  tlie  experiment  of  Sir  W.  Herschel,  in  which  a  point 
of  maximum  heat  was  observed  beneath  the  red  end,  the  physical  independence  of  hght 
and  heat  was  shown,  so  from  analoi^ous  experiments  on  the  chemical  changes  exhibit- 
ed by  the  salts  of  silver,  the  existence  of  a  distinct  class  of  rays,  invisible  to  the  eye, 
designated  "  chemical  rays,"  was  established. 

150.  These  general  results  lead  us,  therefore,  to  suppose  that  there  exist  in  the  so- 
lar beam  a  variety  of  distinct  principles,  and  when  that  beam  is  acted  upon  by  a  prism 
of  glass,  those  principles  are  parted  out  from  each  other.  Among  them  some  are  visi- 
ble, affecting  the  eye  with  the  sensation  of  the  various  colours  of  light,  red,  yellow, 
blue,  &c.,  and  others  are  invisible,  affecting  the  thermometer,  or  producing  chemical 
decompositions.  The  general  idea  which  we  gather  from  these  remarks  is,  that  there 
are  three  separate  principles  coexisting  in  the  solar  ray,  light,  heat,  and  a  principle  of 
chemical  action  ;  and  when  this  ray  is  dispersed  by  a  prism,  three  several  spectra  arise, 
of  which  two  are  invisible,  and  one  can  be  seen.  Their  relative  position  is  such  as  is 
given  inj^o-.  112,  where  A  B  is  the  luminous,  and  therefore  visible  spectrum  ;  C  D  the 
invisible,  chemical  spectrum;  E  F  the  invisible  spectrum  of  heat. 

151.  These  are  the  apparent  phenomena  which  are  exhibited  when  an  ordinary  so- 
lar spectrum  is  employed.  But  in  that  spectrum  the  several  coloured  spaces  are  far 
from  being  pure,  and  many  interesting  phenomena  are  therefore  imperceptible.  New- 
ton, who  has  never  been  surpassed  by  any  experimenter  in  minute  investigation,  studied 
with  great  attention  the  peculiar  circumstances  which  conspire  in  the  production  ot 
the  ordinary  spectrum,  with  a  view  of  isolating  each  one  of  the  coloured  spaces  in  a 
state  of  purity. 

152.  First,  he  shows  that,  when  an  image  of  the  sun  is  formed  in  a  dark  cham- 
ber upon  a  screen,  as  at  z  {fig-  111),  that  image  does  not  terminate  in  a  sharp  circular 
edge,  as  is  the  case  with  the  object  himself,  but  is  surrounded  by  a  penumbra,  the  light 
gradually  fading  away  {fig.  113).  This  can  be  wholly  removed  by  placing  a  convex 
lens  so  as  to  receive  the  ray,  and  give  an  image  in  its  focus.  And  just  as  the  solar  im- 
age is  surrounded  by  the  penumbra!  ring,  so  also  does  the  same  defect  accompany  the 
rays  when  dispersed  by  the  action  of  the  prism,  for  the  faint  light  of  the  penumbra  is 
equally  decomposed  and  dispersed  with  the  bright  light  of  the  central  image.  Such  an 
effect  is  seen  m  fig.  114. 

153.  Suppose,  now,  that  by  the  use  of  a  lens  the  penumbra  is  removed,  and  the  di- 
rect solar  image  {z)  received  on  the  screen  is  seen  to  be  terminated  by  a  sharp  and 
well-defined  edge — the  image,  of  course,  will  be  perfectly  circular.  If,  now,  a  prism  is 
placed  behind  the  lens,  in  such  a  position  as  to  intercept  the  converging  beam  of  light, 
as  at  C  {fig.  115),  where  A  is  the  aperture  in  the  shutter,  B  the  convex  lens  converging 
the  rays  that  come  in  through  the  aperture  to  a  focus,  F,  where  they  form  a  circular 
image  of  the  sun,  the  rays  are  bent  on  one  side  by  the  prism  C,  and  a  neat  spectrum 
is  formed  on  the  screen  at  D  E,  with  sharp  edges,  and  devoid  of  any  penumbral  con- 
tinuation. 

154.  But  the  image  of  the  sun  is  circular,  and  the  spectrum  itself  arises  from  the  sep- 
aration and  successive  superposition  of  coloured  circular  images,  that  separation  being 


46 


PROCESSES  FOR  PURIFYING  THE  SPECTRUM. 


due  to  the  different  refrangibility  of  those  different  coloured  rays.  Let,  therefore,  in 
jig.  116,  royghivhQ^  series  of  such  circular  images,  all  conjointly  composing  the 
spectrum  E  D,  arranged  in  their  proper  order,  r  being  the  red  image,  and  v  the  violet; 
if  we  take  any  one  of  these  images,  such  as  y,  and  consider  its  constitution,  we  shall 
find  that,  by  reason  of  the  overlapping  of  the  adjacent  images,  there  are  rays  in  it 
which  belong  to  the  images  on  either  side,  and  instead  of  the  circle  y  consisting  of 
yellow  light  only,  it  includes  orange  light  derived  from  the  circle  o,  and  green  light 
derived  from  the  circle  g,  as  is  obvious  from  the  overlapping  of  those  circles  o  and  g, 
upon  the  circle  under  consideration,  y. 

155.  Farther,  an  ordinary  spectrum  without  a  penumbra  differs  in  constitution  in  its 
various  parts.  If  a  line,  E  D,  be  drawn  through  its  centre  longitudinally,  the  over- 
lapping of  the  successive  circles  along  that  line  takes  place  to  the  greatest  extent ;  but 
if  another  line,  m  n,  be  drawn  along  its  edge,  on  that  line  the  successive  circles  do  not 
overlap,  and,  therefore,  while  along  the  axis  of  the  spectrum,  E  D,  the  intermixture  of 
the  rays  and  of  the  colours  is  at  a  maximum,  along  the  edges  of  the  spectrum  there  is 
no  overlapping,  no  intermixture,  and  each  one  of  the  coloured  rays  is  in  a  state  of 
purity.  The  light  of  such  a  spectrum,  therefore,  becomes  more  and  more  homogene- 
ous as  we  pass  from  the  axis  and  go  to  the  outer  edge. 

156.  But  suppose  the  total  length  of  the  spectrum  remaining  the  same,  those  circu- 
lar images  are  diminished  in  diameter,  as  fig.  117.  Here,  through  that  diminution 
of  diameter,  overlapping  is  prevented,  and  each  one  of  the  rays  is  separated  out  for  itself, 
there  is  no  superposition,  and  the  colours  are  equally  homogeneous  in  the  axis  of  the 
spectrum  and  on  its  edges. 

157.  This  diminution  in  the  magnitude  of  the  circular  images  may  be  produced  in 
practice  in  several  different  ways.  The  one  which  Newton  recommends,  and  which 
is  generally  adopted,  is  to  use  the  aperture  in  the  shutter  as  the  radiant  source,  instead 
of  the  sun  himself  The  diameter  of  the  sun  subtends  an  angle  of  about  half  a  degree. 
But  over  the  diameter  of  the  aperture  in  the  shutter  we  have,  of  course,  a  complete 
control;  we  can  make  it  half  an  inch,  or  the  hundredth  of  an  inch,  as  experiments  re- 
quire. Instead,  therefore,  of  adjusting  the  screen,  E  D,Jig.  115,  at  such  a  distance  as 
to  receive  the  image  of  the  sun  himself,  we  remove  it  farther  off,  until  we  have  depicted 
upon  it  an  image  of  the  aperture  in  the  shutter  well  defined,  and  with  sharp  edges 
On  interposing  the  prism,  we  find  upon  the  screen,  when  adjusted  to  the  proper  focal 
point,  a  spectrum  of  the  aperture,  in  which  the  separation  of  the  rays  may  be  carried  to 
any  extent  by  diminishing  its  diameter. 

158.  When  this  diminution  is  carried  too  far,  the  quantity  of  light  admitted  becomes 
very  small,  and  under  these  circumstances  Newton  shows  that  great  advantages  arise, 
by  employing,  instead  of  a  circular  hole,  a  longitudinal  slit  or  oblong  aperture;  then  the 
separation  of  colour  is  equally  perfect,  and  more  light  being  admitted,  the  different  ex- 
periments may  be  made  in  a  satisfactory  way. 

159.  Under  certain  circumstances,  it  is  desirable  to  possess  a  spectrum  in  which  the 
intermingling  of  the  colours  by  overlapping  takes  place  in  a  regular  manner.  The  plan 
adopted  by  Newton  was  to  employ  a  triangular  hole.    The  image  of  this,  when  form- 


FIXED  LINES  IN  THE  SPECTRUM. 


47 


ed  by  a  lens  and  dispersed  by  the  action  of  a  prism,  is  as  is  given  in  jig.  118;  where 
the  triangles  overlap  at  the  bases,  A  B,  and  the  intermixture  of  the  different  coloured 
rays  is  at  a  maximum,  but  at  the  other  side  of  the  spectrum  the  vertex  of  each  triangle 
being  separate  from  those  on  either  side,  the  colours  are,  of  course,  in  a  state  of  purity. 

160.  These  are  the  chief  improvements  which  were  introduced  by  Newton  in  the 
formation  of  the  solar  spectrum,  and  it  was  in  one  purilied  in  the  manner  here  descri- 
bed that  his  most  delicate  experiments  were  made.  It  is  extraordinary,  that  after  lie  had 
advanced  so  far  as  to  the  use  of  a  lens,  and  a  narrow  longitudinal  aperture,  that  the 
discovery  of  the  fixed  lines  escaped  him.  Even  if  his  glass  prism  was  imperfect,  these 
lines  are  easily  seen  with  prisms  made  from  pieces  of  looking-glass  cemented  together 
and  filled  with  a  solution  of  sugar  of  lead,  and  such  prisms  Newton  says  that  he  em- 
ployed. 

101.  The  dark  lines  in  the  solar  spectrum  were  first  perceived  by  Dr.  Wollaston, 
and  described  by  Fraunfiofer.  They  may  be  seen  by  forming  on  a  screen  of  w  hite 
card-board, ^o-.  2,  Frontispiece,  the  spectrum  of  a  narrow  fissure  or  chink.  The  lens 
may  be  a  common  convex  lens,  or  an  achromatic;  the  prism  should  be  without  veins 
or  other  flaws  ;  under  these  circumstances,  as  soon  as  the  screen  is  brought  into  the  fo- 
cal position,  so  that  the  spectrum  is  produced  with  sharp  edges,  a  number  of  dark  lines 
will  be  seen  crossing  it  in  certain  positions.  Of  these,  a  sketch  is  given  in^"-.  103,  and 
in  the  frontispiece.  These  different  lines  have  been  designated  by  Fraunhofer  by  the 
letters  of  the  alphabet,  as  follows  :  A  is  in  the  red  region,  B  in  the  red  near  its  outer 
end,  C  is  beyond  the  middle  of  the  red,  D  is  in  the  orange,  and  is  a  double  line,  E  is 
in  the  green,  F  in  the  blue,  G  in  the  indigo,  and  H  in  the  violet. 

162.  Of  the  fixed  lines,  it  is  only  the  larger  which  can  be  seen  by  projection  upon  a 
screen.  Viewed  through  a  telescope,  a  vast  number  of  minute  ones  become  visible. 
Fraunhofer  estimated  them  at  600,  and  these  are  now  known  to  constitute  a  part  only 
of  those  which  actually  exist.  In  the  solar  spectrum,  they  maintain  their  position 
among  the  different  coloured  spaces,  and  the  same  observation  applies  to  the  light  which 
comes  by  reflection  from  any  of  the  planetary  bodies.  In  artificial  lights  they  are  not 
seen;  and  in  the  rays  which  come  from  the  various  fixed  stars,  although  they  are  pres- 
ent, they  are  found  in  different  positions  and  in  different  groups. 

163.  It  has  already  been  stated  that  when  any  salt  of  silver  is  exposed  to  the  spec- 
trum (149),  a  decomposing  effect  takes  place,  and  the  salt  becomes  black.  If  the  spec- 
trum used  be  one  of  a  narrow  fissure,  so  that  the  fixed  lines  are  visible,  it  will  be  found 
that  the  resulting  impression  contains  them  ;  that  where  there  is  a  fixed  line  there  is  a 
corresponding  want  of  action  upon  the  decomposed  surface;  and,  farther,  that  beyond 
the  extreme  violet  end,  and  in  a  region  where  the  eye  can  perceive  no  light,  the  chem- 
ical impression  reveals  remarkable  groups  of  these  inactive  spaces,  M  N  0  V,  Jig.  103. 
These  extra-spectral  lines  were  discovered  about  the  same  time  by  M.  Becquerel  in 
Fiance,  and  by  the  author  of  this  book.  M.  Becquerel's  description  of  them  was, 
however,  published  first.  Their  use  has  become  essential  in  all  chemical  investigations 
connected  with  light. 

164.  Without  anticipating  here  what  will  have  presently  to  be  rtated  in  regard  to 


48 


DISTRIBUTION  OF  HEAT. 


the  chemical  agencies  of  the  solar  beam,  our  attention  is  first  to  be  directed  to  its  cal- 
orific properties.  The  following  facts,  which  are  gathered  from  the  writings  of  M. 
Melloni,  who  has  cultivated  this  department  of  physical  science  with  such  remarka- 
ble success,  will  serve  to  give  an  idea  of  the  opinions  which  have  been  advocated  by 
that  eminent  philosopher. 

165.  The  different  illuminating  power  of  the  different  regions  of  the  spectrum  is,  of 
course,  plainly  evident  to  the  eye.  Newton  supposed  that  the  calorific  effect  of  the 
different  spectrum  regions  followed  the  same  order,  and,  therefore,  that  the  yellow  space 
was  the  hottest.  In  this  view  he  was  followed  by  Landriani,  Sennebier,  Rochon, 
and  others.  But  at  a  later  period,  Sir  W.  Herschel  having  observed  that  when  dif- 
ferent coloured  pieces  of  glass  were  employed  to  intercept  the  solar  rays,  the  amount 
of  heat  which  traversed  them  was  not  in  proportion  to  the  amount  of  light,  he  made 
trials  on  the  solar  spectrum  by  putting  thermometers  in  its  different  colours  and  mark- 
ing their  elevations.  In  the  course  of  these  experiments  he  substantiated  two  remark- 
able facts  :  1st.  That  below  the  red  region,  and  in  a  space  where  no  light  could  be 
seen,  the  thermometer  rose.  2d.  That  tlie  point  of  maximum  temperature  was  not,  as 
Newton  supposed,  in  the  yellow,  but  totally  out  of  tlie  coloured  spaces,  and  among  these 
dark  rays  which  are  less  refrangible  than  the  red  rays  of  light. 

166.  These  experiments  were  repeated  by  various  chemists,  and  with  very  contra- 
dictory results,  some  affirming  the  results  of  Sir  W.  Herschel,  and  others  dissenting 
from  them.  Seebeck,  however,  eventually  proved  that  this  arose  from  the  nature  of 
the  prism  used.  That  when  flint  glass  was  employed,  the  maximum  point  was  in  the 
dark  rays  ;  that  when  crown  glass  was  used,  it  was  at  the  extremity  of  the  red;  for  sul- 
phuric acid  and  alcohol,  it  was  in  the  orange ;  for  water,  it  was  in  the  yellow. 

167.  M.  Melloni  repeated  these  experiments  with  the  aid  of  an  improved  thermom- 
eter, and  established  their  correctness.  He  took  a  prism  of  crown  glass,  which  gave  the 
maximum  of  temperature  at  the  extremity  of  the  red,  and  determined  the  distribution 
of  heat  in  the  coloured  spaces,  and  also  in  the  dark  region  outside  of  the  red.  Then 
he  interposed  a  layer  of  water,  so  that  the  rays  emerging  from  the  prism  were  trans- 
mitted through  it,  and  again  measured  the  temperatures  in  the  obscure  and  the  lumi- 
nous spaces.  They  were  found  altered  ;  some  were  totally  destroyed,  others  partially, 
and  others  had  escaped  without  sensible  change.  The  greatest  absorption  had  taken 
place  among  the  least  refrangible,  the  action  having  been  less  and  less  as  the  red  ray 
was  approached ;  it  diminished  through  the  red,  the  orange,  and  a  part  of  the  yellow, 
and  then  to  the  end  of  the  violet  it  disappeared. 

168.  The  direct  effect  of  this  partial  absorption  is,  of  course,  to  move  the  maximum 
point,  and  bring  it  successively  through  the  red,  the  orange,  and  the  yellow  region.  These 
experiments  also  show  that  the  heat  distributed  through  the  solar  spectrum  is  not  a  ho- 
mogeneous agent,  for  the  same  medium  acts  differently  on  its  differently  refrangible  parts. 
In  the  same  manner,  JNI.  Melloni  showed  that  the  calorific  rays  arising  from  artificial 
sources,  whether  luminous  or  dark,  were  heterogeneous,  and  perfectly  like  the  analo- 
gous rays  existing  in  the  sunbeam.  That  while  most  transparent  substances  had  the 
quality  of  absorbing  some  of  these  rays,  and  allowing  others  to  pass,  there  is  one  body, 


INDEPENDENCK  OF  LIGHT  AND  HEAT.  49 

rock-salt,  which  is  equally  permeable  to  them  all  It  therefore  constitutes  the  true 
glass  of  radiant  heat,  and  glass  and  water  and  other  diaphanous  bodies  act  towards  heat 
as  coloured  media  do  to  light;  thej  possess  for  it  an  invisible  coloration. 

169.  When,  therefore,  we  attempt  to  investigate  the  distril)ution  ol"  heat  in  tlie  solar 
spectrum  by  the  aid  of  a  colourless  flint  glass  prism,  we  are  doing  the  same  thing  as 
if  we  were  attempting  to  study  its  coloured  spaces  by  employing  a  prism  of  blue  or 
green  glass.  Such  an  instrument  would,  of  course,  wholly  disturb  the  proper  constitu- 
tion of  the  spectrum,  absorbing  some  rajs  and  letting  others  pass  ;  and  as  it  acts  towards 
light  so  does  flint  glass  act  towards  heat,  because  it  possesses  an  invisible  coloration 
for  that  principle.  But  rock-salt,  which  is  transparent  equally  to  all  tbese  rays,  will 
not  produce  such  an  effect,  and  hence,  in  all  investigations  connected  with  radiant  heat, 
prisms  of  that  mineral  ought  to  be  employed.  When  this  is  done,  it  is  found  that  the 
niaxin)um  of  temperature  in  this  normal  spectrum  exists  in  the  dark  space,  not  in  con- 
tact with  the  red  extremity,  as  Sir  W.  Herschel  supposed,  but  wholly  detached  from 
the  colours,  at  a  mean  distance  equal  to  that  which  exists  in  the  contrary  direction 
between  the  red  and  the  yellow.  If  the  rays  which  form  this  normal  spectrum  are 
made  to  pass  through  a  plate  of  flint  glass  of  sufiicient  thickness,  the  maximum  ap- 
proaches towards  the  red  region;  if  one  of  ordinary  glass,  it  passes  into  the  red  ;  if  water 
or  alcohol  be  used,  it  enters  the  beginning  of  the  yellow.  But,  by  reason  of  the  limpid- 
ity of  these  different  media,  the  colours  undergo  no  sensible  alteration,  and  the  maxi- 
nmm  remains  always  invariably  fixed  at  the  beginning  of  the  yellow. 

170.  Thus,  the  inferior  bands  of  the  spectrum  may  preserve  the  same  ratio  of  lumi- 
nous intensities,  and  lose  the  relations  which  exist  in  their  temperatures.  The  calorific 
elements  do  not  follow  the  lot  of  their  corresponding  luminous  elements.  Therefore, 
light  and  heat  are  two  different  agents,  or  two  modifications  essentially  distinct  of  one 
common  agent. — (Melloni,  Comptes  Rendus,  Jan.,  1844,  p.  39,  &c.) 

171.  To  these  beautiful  results  of  Melloni,  Sir  J.  Herschel  {Phil.  Trans.,  J  840,  p. 
52)  has  added  other  very  remarkable  ones,  obtained  by  a  different  process.  If  we 
take  a  slip  of  thin  writing  paper,  and,  having  blackened  one  side  of  it  by  exposure  to 
the  smoke  of  a  candle,  adjust  it  so  as  to  receive  the  solar  spectrum,  and  with  a  flat 
brush  equal  in  breadth  to  the  paper  dipped  in  rectified  spirits  of  wine,  wash  over  the 
white  surface  until  the  paper  is  completely  saturated,  and  looks  of  a  uniform  black 
colour,  "  a  whitish  spot  begins  to  appear  below  the  extreme  red  end  of  the  luminous 
spectrum,  which  increases  rapidly  in  breadth  until  it  equals  the  breadth  of  the  luminous 
spectrum,  and  even  somewhat  surpasses  it,  and  in  length  till  it  forms  a  long  appendage 
exterior  to  the  spectrum,  and  extends,  moreover,  within  it,  till  it  reaches  up  to,  and  be- 
yond the  centre  of  the  yellow  ray.  In  this  state,  and  just  as  the  general  drying  of  the 
paper  begins  by  whitening  the  whole  surface  to  confuse  the  appearances,  a  second  sudden 
and  copious  wash  of  alcohol  from  above  downward  must  be  applied,  without  disturbing 
the  spectrum,  or  in  any  way  shaking  the  apparatus.  The  superfluous  alcohol  will  have 
hardly  run  off",  when  the  phenomena  of  the  thermic  spectrum  begin  to  appear  in  all 
their  characters ;  at  first  faintly,  and,  as  it  were,  sketched  in  by  a  dimness  and  dulness 
of  the  otherwise  shining  and  reflective  surface  of  the  wetted  paper ;  but  this  is  speedily 

G 


50 


ACTION  OF  THE  SPECTRUM  OF  THE  DAGUERREOTYPE  PLATE. 


exchanged  for  a  perfect  whiteness,  marking  by  a  clear  and  sharp  outUne  the  lateral  ex- 
tent of  the  calorific  rays,  and  by  due  gradations  of  intensity  in  a  longitudijial  direction 
their  law  or  scale  of  distribution,  both  within  and  without  the  luminous  spectrum." 

172.  "The  most  singular  and  striking  phenomenon  exhibited  in  the  thermic  spec- 
trum thus  visibly  impressed  is  its  want  of  continuity."  It  consists  of  several  distijict 
patches,  or  round  solar  spots,  as  is  seen  in  Jig.  119. 

173.  These  are  the  leading  facts  in  relation  to  the  distribution  of  heat  in  the  New- 
tonian spectrum.  Let  us,  in  the  next  place,  proceed  to  examine  the  laws  of  the  dis- 
tribution and  action  of  the  chemical  rays,  connnencing  first  with  the  decomposition  of 
one  of  the  salts  of  silver,  as,  for  example,  the  iodide  of  silver,  which  forms  the  basis  of 
the  Daguerreotype. 

174.  The  action  of  the  Newtonian  spectrum  on  the  Daguerreotype  plate  is  very  re- 
markable. Imperfect  attempts  were  made,  soon  after  the  preparation  was  published, 
to  determine  the  changes  which  happen  to  this  very  sensitive  substance.  For  the  most 
part,  these,  however,  were  made  without  any  kind  of  precaution  as  respects  the  purity 
of  the  dispersed  colours — precautions  which  Newton,  a  century  ago,  so  clearly  pointed 
out.  Instead,  therefore,  of  spectra  of  great  purity,  in  which  tiie  length  is  many  times 
greater  than  the  breadth,  drawings  were  given  in  which  the  breadth  of  the  solar  image 
is  equal  to  its  length.  Whoever  remembers  that,  in  the  experiments  conducted  by  New- 
ton with  a  view  of  determining  the  phenomena  of  homogeneous  light,  and  in  which 
the  lengths  of  his  spectra  were  sometimes  seventy  times  greater  than  their  breadth,  will 
easily  understand  how  small  must  be  the  value  of  such  imperfect  proofs,  which,  indeed, 
can  scarcely  be  called  impressions  of  the  solar  spectrum. 

175.  The  impressions  obtained  in  July,  1842,  in  Virginia  (A  p.,  645-647),  and  which 
may  be  taken  as  almost  perfect  specimens  of  the  action  of  this  variety  of  spectrum, 
were  made  by  strictly  following  the  precautions  of  Nf.wton  (152,  153).  Several  of 
them  were  obtained,  representing  the  action  under  various  changes  in  the  dimensions 
and  figure  of  the  aperture  admitting  the  light ;  as,  for  example,  when  it  was  a  circle,  a 
triangle,  a  fissure.  Of  the  former  of  these,  a  very  elaborate  account  has  been  given  in 
the  Philosophical  Magazine,  by  Sir  J.  Hi  rschel  {Phil.  Aiag.,  Feb.,  1843),  who  has 
deduced  from  it  what  is  probably  the  true  theory  of  the  action  of  light  on  the  Daguer- 
reotype plate.  In  these  perfect  spectra  there  are  certain  peculiarities  which  cannot  at 
all  times  l)e  produced ;  they  require  an  exceedingly  brilliant  sun,  and  a  clear  sky.  In 
tropical  latitudes  there  is  no  difificulty,  during  the  heat  of  summer,  in  obtaining  them ; 
but  in  New-York,  during  a  great  many  trials  which  I  have  made  in  the  course  of  two 
years,  I  have  but  once  been  able  to  reproduce  them. 

176.  The  solar  spectrum  obtained  upon  iodide  of  silver  may  be  roughly  divided  into 
two  great  and  almost  equal  regions :  the  upper  region,  corresponding  to  the  more  re- 
frangible rays,  embraced  between  the  blue  and  the  extreme  violet,  exhibits,  by  a  deep 
dark  colour,  or  series  of  colours,  that  a  decomposition  of  the  iodide  has  taken  place ; 
probably  in  those  portions  in  which  this  decomposition  has  been  complete,  the  iodide 
has  lost  one  half  of  its  iodine ;  the  lower  region,  which  is  by  far  the  most  interesting 
portion  of  the  two,  and  by  far  the  most  difficult  to  study,  corresponds  to  the  lesser  re- 


ANTAGONIZING  ACTION  OF  THE  TWO  ENDS  OF  THE  SPECTRUM.  51 

frangible  rajs,  which  are  inchided  between  the  extreme  red  and  the  blue.  There  is 
no  difficulty,  at  any  time,  in  securing  the  representation  of  the  upper,  but  it  is  in  this 
lower  part  that  the  diversity  of  result  is  so  great,  and  about  which  there  is  so  much 
difference  of  opinion  among  different  authors. 

177.  I  regard  these  two  classes  of  rays — the  more  and  the  less  refrangible — as  ex- 
hibiting, upon  the  iodide  of  silver,  antagonizing  and  contrary  actions,  the  former  exert- 
ing a  decomposing  agency,  the  latter  a  protecting  agency.  The  circumstances  under 
which  the  lower  region  makes  its  appearance  are  only  found  when  the  sky  has  that 
degree  of  brightness  that  the  sensitive  surface  is  slightly  stained  by  it ;  the  spectrum 
rays  then  exert  their  protecting  agency  on  the  places  on  which  they  fall,  and  maintain 
the  iodide  in  an  undecomposed  state.  That  this  is  the  true  statement  seems  clear  to 
me,  from  the  circumstance  that  the  phenomenon  is  independent  of  time.  It  is  immate- 
rial whether  we  expose  the  sensitive  surface  for  five  minutes  or  for  an  hour,  no  change 
whatever  takes  place  in  this  lower  region  ;  the  iodide  remains  equally  undecomposed. 
Now  this  must  arise  from  the  circumstance  that  the  decomposing  effect  of  the  skylight 
is  exactly  balanced  by  the  protecting  agency  of  the  lower  rays — so  exactly  balanced 
that  it  is  immaterial  whether  the  exposure  be  for  one  minute  or  for  an  hour,  the  result- 
ing action  is  the  same. 

178.  This  view  of  the  relation  between  the  more  and  less  refrangible  rays,  in  the 
action  on  iodide  of  silver,  is  strengthened  by  what  is  known  to  take  place  when  phos- 
phorescent surfaces  are  exposed  to  the  sun.  In  that  case,  as  all  writers  agree,  the  lesser 
refrangible  rays  can  not  only  exert  a  protecting  agency,  but  even  extinguish  the  phos- 
phorescent glow  occasioned  by  those  which  are  more  refrangible. 

179.  Exposed  to  the  prismatic  spectrum,  different  compound  substances  give  rise  to 
very  different  results.  In  some  cases  the  change  is  limited  to  one,  in  others  to  other 
regions  of  the  spectrum.  Thus  the  iodide  of  silver  seems  to  be  decomposed  by  the 
more  refrangible  rays,  the  chloride  exhibits  a  greater  range  of  the  same  species  of  ac- 
tion, the  bromide  is  blackened  by  every  ray  from  end  to  end  of  the  spectrum.  Of  these 
different  results  a  very  extensive  series  has  been  made  known.  They  have  occupied 
much  of  the  attention  of  Sir  J.  Herschel,  who  has  published  the  results  of  his  inves- 
tigations in  the  Transactions  of  the  Royal  Society. 

180.  Fig.  120  shows  such  impressions  on  different  substances,  a  being  the  effect  on 
the  iodide,  and  h  on  the  bromide  of  silver.  In  these  it  is  to  be  remarked,  that  not 
only  does  the  length  of  the  figure  vary  very  greatly,  but  also  the  position  of  the  maxi- 
mum point. 


62 


THE  INTERFERENCE  SPECTRUM. 


CHAPTER  V. 

ON  THE   INTERFERENCE  SPECTRUM. 

Contents  :  Defects  of  the  Prismatic  Spectrum. — Mode  of  forming  the  Interference 
Spectrum. — Its  Peculiarities. —  The  Distribution  of  the  Colours,  and  Law  of  their  In- 
tensities.— Reflected  Interference  Spectrum. — Its  Fixed  Lines. —  Wave-lengths  of  the 
Seven  Great  Rays. 

Mellonis  Researches  on  the  Distribution  of  Heat  in  Perfect  Prismatic  Spectra. — Ap- 
parent  Identity  of  Light  and  Heat. 

Dist?-ibution  of  Chemical  Force  in  the  Interference  Spectrum. — Comparison  of  the  Fix- 
ed Lines  in  the  Prismatic  and  Interference  Spectrum. — Mode  of  Defining  Chemical 
Ejf  'ects  by  Wave-lengths  or  by  Ti?nes  of  Vibration. — Imp>ression  on  Bromide  of  Silver. 
— On  Chloride  of  Silver. —  Total  Change  in  the  Distribution  of  Heat  in  the  Inter- 
ference Spectrum. 

181.  The  prismatic  spectrum  is  obtained  in  a  state  of  the  greatest  purity  when  the 
decomposed  beam  of  light  comes  through  a  narrow  fissure,  and  the  instrumental  ar- 
rangement given  in  (153)  is  employed.  All  the  fixed  lines  are  then  thrown  upon  the 
screen,  and  the  separation  of  the  different  homogeneous  rays  is  accomplished,  perhaps, 
as  perfectly  as  is  possible. 

182.  But  in  this  spectrum,  as  employed  for  investigating  the  chemical  action  of  the 
solar  beam,  very  serious  difficulties  may  be  traced.  If  we  inspect  a  photographic  im- 
pression, or  if  we  consider  the  intensity  of  the  luminous  rays  in  its  different  parts,  we 
perceive  that,  as  the  violet  end  is  approached,  the  different  changes  take  place  in  a  less 
abrupt  manner;  the  light  fades  gradually  and  imperceptibly  away,  the  photograph  does 
not  end  sharply,  but,  spreading  itself  out  to  a  great  distance,  disappears  so  gradually 
that  we  can  scarcely  say  where  its  termination  is,  and  the  fixed  lines  increase  in  num- 
ber and  breadth  compared  with  what  we  perceive  at  the  red  extremity. 

183.  These  different  results  obviously  arise  from  an  inherent  defect  in  the  prismatic 
spectrum  ;  a  defect  which  originates  in  the  very  cause  which  gives  rise  to  the  spectrum 
itself — unequal  refrangibility.  Of  two  sets  of  rays  compared  together,  one  set  taken  in 
the  red,  and  the  other  in  the  violet  region,  it  is  obvious  that,  in  the  same  spectrum, 
from  the  very  circumstance  of  their  greater  refrangibility,  those  in  the  violet  will  be 
relatively  more  separated  from  each  other  than  those  in  the  red.  That  is  to  say,  if  we 
have  two  rays,  a  and  b,  in  the  red,  whose  indices  of  refraction  are  almost  identical,  and 
which,  projected  in  the  spectrum,  would  stand  close  to  each  other,  side  by  side,  and 
another  pair,  c  and  d,  in  the  violet,  whose  indices  of  refraction  bear  precisely  the  same 
relation  to  one  another,  these  last,  by  the  act  of  refraction,  will  be  more  separated,  be- 
cause they  are  relatively  more  refrangible  than  the  former.  The  result  of  this  increased 
separation  in  the  more  refrangible  regions  is  to  give  an  apparent  dilution  to  them,  while 


MODE  OF  FORMING  THE  INTERFERENCE  SPECTRUM.  53 

the  lesser  refrangible  regions  are  concentrated.  The  relative  position  of  the  colours 
must  also  vary;  the  fixed  lines  must  be  placed  at  distances  greater  than  the  true  dis- 
tance, as  the  violet  end  is  approached. 

184.  In  all  investigations  on  the  chemical  action  of  the  spectrum,  the  greatest  impor- 
tance is  to  be  attached  to  these  considerations,  for  our  estimates  of  chemical  results  de- 
pend on  the  amount  of  action  taking  place  in  a  given  time.  When,  therefore,  we  ob- 
tain a  prismatic  impression  on  any  sensitive  surface,  it  is  very  far  from  representing  the 
true  character  of  the  phenomena.  The  action  which  ought  to  be  concentrated  in  a 
lesser  space  at  the  more  refrangible  region  is  spread  over  a  greater,  and  with  that  aug- 
mentation an  apparent  diminution  of  the  amount  of  action  is  perceived.  1  his,  of 
course,  should  make  the  maximum  point  vary,  spread  out  unduly  the  violet  end,  and 
dilute  the  true  effect.  The  different  regions  of  the  prismatic  spectrum  cannot  be  fairly 
compared  with  one  another. 

185.  My  attention  having  been  directed  to  these  peculiarities,  I  obtained  from  the 
mint  at  Philadelphia,  in  May,  1843,  a  piece  of  ruled  glass,  with  a  view  of  examining 
the  different  questions  for  which  solutions  had  been  apparently  obtained  by  the  defec- 
tive use  of  the  prismatic  spectrum,  substituting  for  it  the  interference  spectrum  of  Fraun- 
HOFER.  I  am  particular  in  mentioning  this  date,  because  I  perceive  that,  at  the  very 
time  that  I  was  making  these  trials  in  New-York  on  the  chemical  agencies  of  the  s])ec- 
trum,  Professor  Masotti  was  doing  the  same  thing  for  the  luminous  rays  in  Italy,  as  is 
stated  by  M.  Melloni,  in  the  Comptes  Rendus,  Jan.,  1844,  p.  44;  and  it  is  possible 
that,  before  the  time  that  this  volume  reaches  the  Continent  of  Europe,  other  skilful  ex- 
perimenters may  have  followed  in  the  course  of  Professor  Masotti,  and,  perceiving  the 
great  advantages  which  arise,  may  have  applied  it  for  the  chemical  rays. 

186.  As  the  interference  spectrum  is  less  known  than  the  prismatic,  I  will  here  give 
a  brief  account  of  the  mode  by  which  it  is  to  be  formed  for  experimental  purposes,  and 
of  its  peculiarities. 

187.  The  principal  apparatus  is  a  grating  composed  of  a  number  of  intervals,  alter- 
nately transparent  and  opaque.  A  piece  of  glass,  with  lines  ruled  on  it  with  the  point 
of  a  diamond,  forms  the  best  grating ;  these  lines  should  be  parallel  to  one  another,  equi- 
distant, and  so  close  that  several  hundreds,  or  even  thousands  of  them,  may  be  com- 
prised in  an  inch.  To  so  great  a  degree  of  perfection  did  Fraunhofer  carry  this  kind 
of  work,  that  he  obtained  gratings  containing  30,000  lines  in  the  inch.  A  beam  of 
light,  directed  horizontally  into  a  dark  room  by  means  of  a  heliostat,  is  to  be  trans- 
mitted through  a  fissure,  as  in  the  arrangement  for  showing  the  fixed  lines  in  the  pris- 
matic spectrum.  At  a  distance  of  several  feet,  the  ray  is  received  on  the  ruled  glass, 
behind  which,  a  few  inches  intervening,  a  piece  of  ground  glass  is  placed.  The  lines 
which  are  upon  the  glass  are  to  be  adjusted  so  that  they  are  parallel  to  the  sides  of  the 
fissure  through  which  the  light  enters.  In  Fraunhofer's  arrangement,  instead  of  a 
ground  glass,  a  small  achromatic  telescope  was  placed,  having  a  mechanism  for  meas- 
uring angular  deviations,  and  rotating  on  an  axis  coincident  with  the  centre  of  the  gra- 
ting ;  the  following  facts,  which  may  less  exactly  be  verified  with  a  ground  glass,  were 
then  determined.    Upon  the  axis  of  the  ray  coming  in  through  the  fissure,  a  white 


54 


DESCRIPTION  OF  THE  SUCCESSIVE  SPECTRA  AND  THEIR  LINES. 


image  of  that  fissure  was  seen ;  its  sides  were  perfectly  sharp,  and  of  the  same  appear- 
ance as  though  the  grating  had  not  intervened ;  on  the  right  and  on  the  left  of  this 
image,  two  equal  spaces,  completely  dark,  and,  beyond  each  of  these  spaces,  a  series 
of  solar  spectra,  each  having  its  violet  extremity  pointing  inward  towards  the  central 
image,  and  its  red  extremity  outward.  Of  these  spectra,  the  first  on  each  side  is  per- 
fectly insulated,  but  the  violet  of  the  third  projects  upon  the  red  of  the  second,  and  in 
the  same  way  each  successive  spectrum  is  overlapped  by  those  coming  after  it.  These 
spectra  are  situated  symmetrically  on  each  side  of  the  central  image,  and  with  a  tele- 
scope, or  when  other  proper  means  are  used,  the  fixed  lines  are  plainly  visible  in  them. 

188.  In  jig.  121  these  phenomena  are  represented :  A  is  the  central  white  image ; 
BCD....  the  successive  spectra  on  the  right  hand  ;  E  F  G,  the  symmetrical  ones  on 
the  left. 

189.  If,  now,  we  measure  the  distance  of  any  one  of  the  fixed  lines,  for  example  H, 
from  the  middle  of  the  central  image  in  the  successive  spectra  on  one  side,  we  shall 
find  that  the  distances  which  separate  that  centre  from  this  ray  in  the  consecutive  spec- 
tra on  one  side — H'  in  the  first,  H"  in  the  second,  H'"  in  the  third  spectrum — are  in 
an  arithmetical  progression  ;  A  H"  is  double,  A  H'"  is  triple  of  A  H'.  The  angular  sep- 
aration, measured  by  the  instrument,  between  a  given  ray  and  the  middle  of  the  cen- 
tral image,  is  termed  the  deviation. 

190.  "  Fraunhofer  has  measured  thus  the  deviations  of  the  principal  lines,  making 
use  of  different  gratings  ;  that  is  to  say,  of  gratings  the  lines  of  which  were  more  or  less 
close,  and  in  which  the  opaque  groove  was  more  or  less  large  compared  with  the  trans- 
parent interval.  He  has  proved,  in  this  manner,  that  the  deviation  of  the  same  line,  or 
of  the  same  colour,  does  not  depend  on  the  ratio  of  the  width  of  the  groove  to  the 
width  of  the  transparent  interval,  but  on  the  sum  of  those  two  magnitudes;  that  the  abso- 
lute value  of  the  deviation  is  in  the  inverse  ratio  of  this  sum ;  that  is  to  say,  that  in 
multiplying  the  measured  deviation  by  the  known  sum  of  the  width  of  a  groove  and  a 
transparent  interval,  we  obtain  a  number  which  is  constant  for  the  same  ray,  whatever 
may  be  the  grating  which  we  use.  Fraunhofer  has  calculated  upon  exact  and  numer- 
ous experiments  the  values  of  this  constant  for  the  seven  principal  lines  of  the  solar 
spectrum,  and  finds  that  these  values  are  precisely  equal  to  the  lengths  of  undulations  of 
colours  corresponding  to  those  rays,  such  as  Fresnel  had  obtained  by  other  processes." 
— (Lame'.) 

191.  Of  these  phenomena  the  undulatory  theory  gives  a  rigorous  account.  It 
farther  shows,  that  the  deviation  of  one  of  the  colours  in  the  first  spectrum,  multiplied 
by  the  sum  of  the  breadth  of  one  of  the  grooves  of  the  grating,  and  of  its  correspond- 
ing transparent  part,  is  equal  to  the  length  of  an  undulation  of  that  particular  colour. 
This  product,  therefore,  is  constant  for  all  gratings,  and  gives  an  easy  means  "of  de- 
termining the  length  of  an  undulation. 

192.  In  the  same  spectrum,  also,  the  deviations  of  any  two  colours  are  to  one  an- 
other as  the  lengths  of  their  respective  undulations.  For  (his  reason,  therefore,  the 
violet  ray  is  situated  nearest  to,  and  the  red  farthest  from  the  ()))tical  axis. 

,193.  From  a  note  attached  to  M.  Melloni's  memoir  {Conqites  Rendus,  Jan.,  1844, 


I 


THE  INTERFERENCE  SPECTRUM  BY  REFLEXION.  55 

p.  43),  it  appears  that  Professor  Masotti  has  determined  that  in  these  beautiful  spectra 
the  most  himinous  point  is  in  the  centre  of  the  yellow  ray,  which  is  itself  placed  at  an 
equal  distance  from  each  extremity  of  the  spectrum.  From  this  central  point  the  lu- 
minous intensity  declines  regularly  and  equally  to  each  end,  so  that  the  red  limit  and 
the  violet  limit  have  both  the  same  luminous  intensity,  and  are  the  least  luminous  parts 
of  the  spectrum.  M.  Masotti  has  also  shown  that  the  colours  of  these  two  limits  are 
formed  by  ethereal  waves,  the  lengths  of  which  are  respectively  to  each  other  in  the 
remarkable  proportion  of  2  :  1.  On  the  frontispiece  the  interference  and  prismatic 
spectra  are  compared  side  by  side. 

194.  Before  directing  farther  attention  to  the  peculiarities  of  the  interference  spec- 
trum, I  shall  proceed  to  explain  the  method  of  obtaining  it  which  I  have  found  most 
suitable  for  chemical  purposes.  Through  a  narrow  fissure.  A,  Jig.  4,  frontispiece,  I 
direct  a  beam  of  light  horizontally  by  a  heliostat,  and  at  a  distance  of  twelve  feet  I 
receive  it  upon  the  surface  of  a  piece  of  ruled  glass,  the  lines  of  which  are  parallel  to 
each  other,  and  rigorously  parallel  to  the  heliostat  fissure,  A.  More  than  a  year  ago  I 
found  that  there  are  practically  great  advantages  in  using  a  reflecting  grating  over  one 
in  which  the  light  is  directly  transmitted.  The  glass,  B  C,  is  therefore  to  be  silvered 
with  a  piece  of  tin  foil,  so  as  to  act  like  a  mirror,  and  it  will  be  found  that  the  tin 
amalgam  upon  it  copies  perfectly  all  the  ruled  lines.  In  this  mode  of  operating  there 
is,  therefore,  no  difficulty  in  placing  B  C  so  that  the  ray  coming  froui  A  falls  perpen- 
dicularly on  it,  for  all  that  is  required  to  ensure  the  proper  adjustment  is  to  move  B  C 
until  it  is  in  such  a  position  that  the  ray,  after  reflexion  by  it,  returns  back  to  the  heli- 
ostat fissure,  A.  At  a  distance  from  B  C  of  six  inches,  I  place  an  achromatic  object 
glass,  D,  in  such  a  position  that  it  shall  receive  perpendicularly  the  reflected  rays  of  the 
spectrum  of  the  first  order ;  in  my  arrangement  this  happens  to  be  the  left-hand  spec- 
trum, as  the  observer  looks  towards  the  heliostat  fissure,  A.  The  achromatic  lens  is 
thus  brought  as  near  to  the  grating  as  possible,  without  its  edge  intercepting  the  ray 
coming  from  A.  In  the  focus  of  this  lens,  at  E  F,  a  ground  glass  is  placed  ;  it  is  adjust- 
ed in  a  box,  E  G,  and  so  arranged  that  the  ground  glass  may  be  removed,  and  any 
photogenic  surface  put  in  its  place.  This  portion  of  my  apparatus  is,  however,  nothing 
more  than  the  sliding  part  of  a  common  Daguerreotype  camera,  which  contains  the  ne- 
cessary ground  glass  and  cases  for  photogenic  preparations. 

195.  The  apparatus  being  properly  adjusted,  so  that  the  ground  glass  receives  the 
rays  perpendicularly,  the  spectrum  will  be  seen  depicted  upon  it  in  beautiful  perfection, 
exhibiting  all  its  striking  peculiarities  and  its  fixed  fines  quite  visible.  Every  precau- 
tion should  be  used  to  shut  out  all  extra ueous  light. 

196.  The  descriptions  given  of  the  interference  spectrum  by  optical  writers  refer 
to  the  transmitted  spectrum  ;  but  these  which  are  obtained  by  reflexion  are,  as  Fraun- 
HOFER  showed,  like  those  in  all  respects.  For  several  reasons,  it  would  be  preferable 
to  use  parallel  lines  ruled  on  a  piece  of  polished  steel  or  speculum  metal.  Silvered 
glass,  however,  has  the  advantage  of  not  changing  readily  under  the  influence  of  the 
various  corrosive  vapours  used  in  these  researches.  The  grating  which  I  employ  is  five 
eighths  of  an  inch  long,  and  one  third  of  an  inch  broad. 


56 


WAVE  LENGTHS  OF  THE  SEVEN  GREAT  RAYS. 


197.  The  interference  spectrum  is  given  in  Jig.  133  and  in  the  frontispiece,  with  its 
fixed  lines  and  coloured  spaces,  as  seen  on  the  ground  glass.  By  the  side  of  it  is  placed  the 
prismatic  spectrum,  the  spectrum  of  Newton.  Compared  together,  we  see  that  in  the 
latter  the  yellow  region  is  among  the  less  refrangible  rays,  in  the  former  it  occupies 
the  central  position  of  the  spectrum.  The  distribution  of  the  fixed  lines  is  also  differ- 
ent, though  their  position,  as  respects  the  coloured  spaces,  is  the  same  for  both  spectra. 
Of  course  the  light  is  distributed  differently  in  respect  to  its  intensity  in  the  two  spec- 
tra, as  has  been  said:  for  the  one  it  is  a  maximum  in  the  centre,  and  fades  away  equally 
to  each  extremity ;  but  for  the  other,  the  illuminating  power  is  as  given  in  the  table 
(147)  derived  from  Fraunhofer. 

198.  The  following  table,  derived  from  Sir  J.  Herschel's  Treatise  on  Light,  gives 
the  lengths  of  the  waves  of  light  corresponding  to  each  one  of  the  principal  lines  of  the 
spectrum.  The  Paris  inch  is  supposed  to  be  divided  into  one  hundred  millions  of  equal 
parts,  one  of  which  constitutes  the  unit  of  the  scale. 

Length  of  wave  corresponding  to  the  ray  B,  2541 

C,  2422 

«  "        D,  2175 

«  "        E,  1945 

«<  «  «        Y,  1794 

*•  "  «        G,  1587 

H,  1464 

199.  In  the  preceding  chapter,  we  have  given  a  brief  account  of  the  experiments  of 
M.  Mi  LLONi  on  the  distribution  of  heat  in  the  spectrum,  and  shown  (170)  how  he  de- 
duced from  those  results  the  doctrine  of  the  physical  independence  of  the  two  im- 
ponderable principles.  More  recently,  however,  M.  Melloni  has  repeated  the  same 
experiments  {Comptes  Rendus,  Jan.,  1844),  and  draws  from  them  a  result  diametrically 
opposed  to  the  former. 

200.  That  a  philosopher  should,  with  the  progress  of  knowledge,  change  his  opin- 
ions on  fundamental  points  of  science,  is  very  far  from  being  a  fault.  For  more  ilian 
ten  years  M.  Mklloni  has  studied,  with  unexampled  success,  the  laws  and  phenomena 
of  radiant  heat.  The  leading  doctrine  which  he  has  deduced  from  those  researches  is 
tliat  to  which  we  have  just  referred,  the  entire  pliysiai I  hidependeiice  of  light  and  Jteat. 
From  recent  repetitions  of  his  former  experiments,  he  has  now  arrived  at  a  conclusion 
apparently  opposite,  "  that  light  is  only  a  series  of  calorific  indications  sensible  to  the 
organ  of  sighr,  or,  vice  versa,  that  the  radiations  of  dark  heat  are  true  invisible  radia- 
ti'jns  of  light."  The  arguments  which  have  been  employed  (169)  in  discussing  the 
experiments  of  Seebeck  and  others,  on  the  shifting  of  the  maximum  point  when  dif- 
ferent diaphanous  media  are  used  from  the  red  into  the  orange,  or  even  into  the  yellow, 
all  hang  upon  the  admission  that  the  spectrum  under  consideration  is  sufficiently  pure, 
and  that  there  is  no  superposition  or  overlapping  of  its  parts.  For,  were  this  the  case, 
even  where  colourless  and  diaphanous  media  are  used,  such  as  crown-glass,  water,  sul- 
phuric acid,  and  alcohol,  though  these,  by  reason  of  their  transparency,  do  not  exert 
any  absorbent  action  on  the  rays  of  light,  they  might,  by  reason  of  their  invisible  col- 


DISTRIBUTION  OF  HEAT  IN  A  PURE  SPECTRUM.  57 

oration,  absorb  certain  of  those  rays  which  are  bejond  the  extreme  red ;  and  if  these 
obscure  rays,  by  superposition  or  overlapping,  extended  from  their  proper  places  into 
the  coloured  regions  above,  the  absorptive  action  of  the  diaphanous  media  would  then 
be  exercised,  and  apparent  movements  of  the  point  of  maximum  take  place,  such  as  have 
been  actually  observed — the  point  shifting  successively  from  the  obscure  spaces  into  the 
red,  from  the  red  to  the  orange,  from  the  orange  into  the  yellow. 

201.  Now,  in  those  spectra  which  were  formerly  employed,  both  by  Sir  W.  Herschel 
and  M.  Melloni,  this  very  difiiculty  must  have  taken  place,  and  the  results  obtained 
were  therefore  not  simple,  but  compound.  It  is  obvious,  if  we  would  avoid  these  in- 
terferences, that  we  must  employ  a  spectrum  capable  of  showing  the  fixed  lines,  the 
parts  of  which  are  properly  separated  from  one  another, 

202.  On  making  use  of  such  a  spectrum,  M.  Melloni  found  the  maxinnuii  temper- 
ature uniformly  at  the  extremity  of  the  red  ray,  it  being  immaterial  what  diapha- 
nous substance  had  been  used  as  a  prism  or  as  an  absorbent  medium.  When  glasses 
of  a  brownish  colour  were  interposed,  a  colour  which  results  from  their  exerting  an  ab- 
sorbent action  on  all  the  colours  of  the  purified  spectrum,  it  appeared  that  so  intimately 
were  "  these  colours  alhed  to  their  temperatures,  that  during  the  transmission  they  lost 
as  much  heat  as  light,  so  that  the  ratio  of  these  two  agents  ren)ained  always  unaltera- 
ble." From  these  things  it  is  deduced,  "That  the  luminous  radiations,  disengaged  from 
every  other  heterogeneous  radiation,  have  a  heat  of  their  own,  which  follows  exactly 
the  same  vicissitudes,  so  that  the  different  phases  of  a  given  ray  of  simple  light  may  be 
measured  indififerently  by  its  luminous  or  calorific  relations." 

203.  Of  all  the  departments  of  physical  science,  there  is  none  about  which  so  many 
experimental  difficulties  gather  as  that  which  is  connected  with  investigations  on  the 
solar  spectrum,  which  is  the  point  of  reunion  of  the  most  energetic  and  powerful  im- 
ponderable agents,  visible  and  invisible  ;  and  probably  we  shall  not  obtain  true  views 
of  its  constitution  and  properties  until  our  doctrines  have  changed  over  and  over  again, 
and  years  have  elapsed.  In  the  views  just  cited,  M.  Melloni  returns  to  the  opinions 
held  at  the  beginning  of  this  century ;  those  views  which  led  to  the  invention  of  Les- 
lie's photometer. 

204.  That  some  splendid  generalization  will  hereafter  unite  all  these  imponderable 
principles,  we  have  repeatedly  said ;  but  there  are  very  many  facts  now  known  which 
none  of  the  views  hitherto  brought  forward  can  embrace.  Under  these  circumstances, 
it  would  seem  that  the  proper  course  to  pursue  is  to  regard  each  one  of  these  agents  as 
physically  distinct.  Notwithstanding  the  clearness  with  which  M.  Melloni  has  put 
forth  his  recent  doctrines,  those  who  have  read  with  attention  the  beautiful  memoirs 
which  he  has  formerly  written,  will  pause  before  they  assent  unconditionally.  It  is  not 
by  a  difficult  and  delicate  experiment,  such  as  that  now  under  consideration,  nor  even 
by  a  series  of  them,  that  that  assent  will  be  readily  obtained.  There  are  former  exper- 
iments to  be  explained  away,  former  measures  to  be  accounted  for,  before  this  desirable 
simplicity  can  be  effected.  It  is  very  true  that,  admitting  the  doctrine  of  the  identitv  of 
light  and  heat,  or  their  common  dependance  on  one  higher  agent,  the  experiment  we  have 
just  described  meets  with  a  ready  explanation ;  but  we  become  involved  in  difficulties 

H 


•58 


MELLONI'S  EXPERIMENT  OF  LIGHT  WITHOUT  HEAT. 


when  we  attempt  similar  explanations  for  other  facts.  Thus,  a  thin  plate  of  transparent 
alum  and  a  thick  mass  of  smoky  rock  crystal,  the  former  completely  diaphanous,  the  lat- 
ter so  opaque  that  even  when  placed  in  the  fullest  light  it  was  not  possible  to  read  large 
printed  characters,  being  compared  together,  the  latter  was  found  to  be  more  transparent 
to  heat  than  the  former  in  the  proportion  of  19  to  6;  and,  indeed,  if  these  agents  be  iden- 
tical, and  the  rays  of  light  have  a  proper  heat  of  their  own,  so  that  their  amount  may 
be  measured  by  a  thermometer,  what  becomes  of  such  experiments  as  the  following, 
brought  forward,  on  the  other  side  of  the  question,  by  JNI.  Melloni,  formerly,  who  pass- 
ed a  ray  of  light  through  a  stratum  of  water,  and  then  through  a  glass  tinged  green  by 
the  oxide  of  copper?  "  The  pure  light  emerging  from  this  system  contains  much  yel- 
low, and  possesses,  at  the  same  time,  a  tinge  of  bluish  green.  If  exhibits  no  calorific 
action  capable  of  being  rendered  perceptible  by  the  most  delicate  thermoscopes,  even  when 
it  is  so  concentrated  by  lenses  as  to  rival  the  direct  rays  of  the  sun  in  brilliancy." — (  Tay- 
lor's Scientific  Memoirs,  vol.  i.,  p.  392. 

205.  Until,  therefore,  an  explanation  can  be  given  of  this  experiment,  or  its  authenticity 
disproved,  the  recognised  rules  of  chemistry  require  that  we  should  speak  of  light  and 
heat  as  distinct  agents.  Nor  are  these  the  only  imponderable  principles  which  are  in- 
volved ;  there  are  others  known  to  exist,  the  position  of  which  is  determined  by  these 
discussions:  the  chemical  rays,  for  example,  which  M.  Becquerel  regards  as  nothing 
more  than  invisible  light,  adopting  for  them  the  same  theory  which  M.  Melloni  has 
adopted  for  the  rays  of  heat.  There  are  also  rays  which  can  excite  the  glow  from  phos- 
phorescent bodies.  From  the  time  of  Franklin  and  Canton  it  has  been  known,  that 
if  calcined  oyster  shells  were  exposed  to  the  rays  of  an  electric  spark,  as,  for  example, 
to  the  discharge  of  a  Leyden  vial,  the  shells  would  commence  to  shine ;  but,  as  may 
easily  be  proved,  it  has  more  recently  been  found,  that  if  the  rays  thus  exciting  the 
phosphorescent  quality  be  transmitted  through  a  piece  of  colourless  flint  glass,  they 
can  no  longer  produce  the  result.  It  might  be  said  that  this  arises  from  the  fact  that 
colourless  glass  has  an  invisible  coloration,  and  that  it  absorbs  the  rays  which  are  far 
beyond  the  violet,  but  direct  experiment  proves  that  flint  glass  possesses  no  such  quality. 

206.  Inasmuch,  therefore,  as  a  piece  of  flint  glass  can  cut  off' those  dark  rays  which 
excite  phosphorescence  in  the  sulphuret  of  lime,  and  combinations  such  as  that  which 
has  been  referred  to  (204)  are  known,  which  can  separate  light  from  heat,  we  are  jus- 
tified in  regarding  these  as  distinct  imponderable  principles.  They  may,  it  is  true,  be 
remotely  identical,  but  they  are  not  identical  in  the  way  here  set  forth.  They  may 
all  consist  of  similar  movements  or  undulations  of  one  primordial  ether,  but  there  are 
points  in  which  the  mechanism  of  those  undulations  vary — circumstances  which  im- 
press upon  them  physical  differences.  As,  in  the  phenomena  of  sound,  we  may  have 
instruments  which  give  rise  to  undulations  of  the  same  length  and  the  same  intensity 
— instruments  which  are  executing  the  same  strain  of  music  at  the  same  time — yet  we 
know  well  enough  that  each  one  impresses  its  own  proper  modifications,  which  the  ear 
in  an  instant  detects.  At  a  distance,  we  recognise  the  flute,  the  viohn,  the  piano,  the 
bugle,  from  one  an  t'  er. 

207.  The  imperfections  of  the  prismatic  spectrum,  which  have  been  just  described 


CHEMICAL  EFFECTS  DESCRIBED  BY  WAVE-LENGTHS  AND  TIMES  OF  VIBRATION.  59 

as  exciting  so  powerful  an  influence  in  the  distribution  of  light  and  heat,  are  equally 
perceptible  in  the  case  of  chemical  effects,  a  result  which  becomes  apparent  at  once 
when  we  operate  with  the  interference  spectrum.  When,  on  any  sensitive  silver  sur- 
face, this  spectrum  is  received,  instead  of  a  stain  exceeding  in  length  the  coloured  spa- 
ces, the  change  is  limited  to  a  narrow  region,  occupied  by  the  more  refrangible  rays. 

208.  In  Jig.  133,  the  visible  interference  spectrum  is  given  with  its  fixed  Unes,  as  it 
appears  on  the  ground  glass  of  the  camera.  The  figure  in  the  frontispiece  is  drawn 
from  the  measures  of  Fraunhofer.  If  the  two  be  compared  together,  it  will  be  seen 
that  there  are  difierences  in  the  relative  distances  of  the  lines.  These  differences  arise 
from  the  mode  of  conducting  the  experiment  in  the  two  cases.  Fraunhofer's  spec- 
trum was  carefully  determined,  the  angular  deviations  being  measured  on  a  graduated 
arc.    Fig.  133  is  depicted  from  the  appearance  on  the  flat  ground  glass  of  the  camera. 

209.  Before  giving  a  description  of  the  chemical  effects  of  this  spectrum,  it  is  neces- 
sary to  explain  the  method  of  subdividing  the  spectrum  which  is  here  resorted  to.  In 
the  earlier  discussions  on  the  chemical  effects  of  light,  the  different  regions  of  the  spec- 
ti'um  were  marked  out  by  the  designations  of  the  different  coloured  rays,  and  effects 
were  described  as  taking  place  in  the  red,  or  yellow,  or  violet  ray.  An  improved  plan 
was  proposed  by  Sir  J.  Herschei.,  and  followed  by  him  in  his  various  writings  :  it 
consists  essentially  in  dividing  the  space  which  exists  between  the  red  and  yellow  ray, 
as  insulated  by  cobalt  blue  glass,  into  13-30  parts,  taking  the  centre  of  the  yellow  ray  as 
the  zero  point,  and  continuing  the  divisions  equally  into  the  more  and  less  refrangible 
regions. 

210.  Over  these  different  methods  the  use  of  the  fixed  lines  possesses  very  great  ad- 
vantages, inasmuch  as  we  do  not  make  reference  to  ideal,  but  to  actually  visible  points 
existing  in  the  spectrum.  Since  the  discovery  of  these  lines,  therefore,  both  M.  Bec- 
QUEREL  and  myself  have  used  them  to  mark  out  spectrum  regions.  The  only  difficul- 
ty which  is  in  the  way  is,  that  they  do  not  give  subdivisions  minute  enough  for  many 
purposes.  But  this  difficulty  can  be  wholly  removed,  and  other  very  great  advantages 
gained,  by  using  them  in  the  manner  which  I  shall  now  proced  to  explain. 

211.  It  has  been  stated  that  the  deviations  (189)  of  the  different  fixed  lines,  B,  C,  D, 
in  the  interference  spectrum,  are  proportional  to  the  lengths  of  the  undulations  which 
they  respectively  represent.  By  designating  the  different  points  of  the  spectrum  by 
their  wave-lengths,  the  subdivision  may  be  carried  to  any  degree  of  minuteness ;  the 
measures  of  one  author  will  compare  with  those  of  another,  and  the  different  phenom- 
ena of  chemical  changes  occurring  through  the  agency  of  light  becon)e  allied  at  once 
with  a  multitude  of  other  optical  results  ;  as,  for  example,  when  we  are  told  that  the 
decomposition  of  bromide  of  silver  occurs  at  a  maximum  under  the  influence  of  a  ray 
which  is  0-00001538  of  a  Paris  inch  in  length,  we  recognise  in  an  instant  that  this  ray 
falls  between  the  fixed  lines  G  and  H,  the  length  of  G  being  0-00001587,  and  of  H 
0-00001464;  we  see,  moreover,  that  the  point  spoken  of  is  nearer  to  G  than  to  H,  and,  if 
it  were  necessary,  by  a  very  simple  arithmetical  process,  we  could  determine  the  number 
of  vibrations  executed  by  that  ray  in  thus  bringing  about  the  maximum  decomposition  in 
billionths  of  a  second,  using  the  rate  of  the  propagation  of  light  at  192,000  miles. 


60 


CHEMICAL  EFFECTS  OF  THE  INTERFERENCE  vSPECTRUM. 


212.  The  fixed  lines,  used  in  this  way,  enable  us  at  once  to  divide  the  interference 
spectrum  into  any  number  of  parts,  and  to  indicate  effects  either  in  space  or  in  time. 
For  chemical  purposes,  in  which  mathematical  accuracy  is  scarcely  attainable,  all  that 
we  have  to  do,  in  order  to  determine  the  lengths  of  waves  producing  given  effects,  or 
the  times  of  vibration,  is  to  determine,  upon  the  interference  spectrum,  the  point  at 
which  the  change  in  question  has  taken  place,  and,  using  the  numbers  which  Fraun- 
HOFER  measured  for  the  different  fixed  lines,  find  what  is  its  relation  to  them.  Thus, 
for  example,  suppose  it  has  been  found,  by  experiment,  that  a  certain  substance,  expo- 
sed to  the  interference  spectrum,  exhibited  a  maximum  point  of  decomposition  exactly 
midway  between  the  fixed  lines  F  and  G,  which  are  both  impressed  on  it,  it  is  required 
to  know  what  is  the  length  of  the  wave  which  brought  about  that  decomposition  in 
parts  of  a  Paris  inch.    The  numbers  given  by  Fraunhofer  are. 

For  G   0-00001587, 

"    F   0.00001794; 

and  the  point  in  question,  being  midway  between  the  two,  has  for  its  wave-length 
0-00001690. 

213.  In  the  frontispiece,  I  have  annexed  Fraunhofer's  numbers  to  each  ray,  omit- 
ting, for  the  sake  of  brevity,  the  ciphers. 

214.  In  Jig.  134  is  represented  the  effect  of  the  interference  spectrum  upon  a  silver 
plate,  rendered  sensitive  by  exposure  to  iodine  vapour,  and  then  to  bromine.  The  time 
of  its  exposure  in  the  camera  was  half  an  hour.  The  point  of  maximum  falls,  as  may 
be  determined  upon  the  foregoing  principles,  nearly  at  the  point  0-00001538.  The 
dotted  lines,  x  and  y,  indicate  the  beginning  and  end  of  the  stain.  The  maximum 
point  does  not  fall  equally  between  the  two,  but  is  nearer  to  the  more  refrangible  ex- 
tremity. When  this  spectrum  is  compared  with  the  corresponding  prismatic  one,  in 
Jig.  103,  we  see  how  great  is  the  difference  ;  the  effect,  which  is  there  carried  far  be- 
yond the  extra-violet  regions,  is  here  compressed  down  into  a  narrow  space. 

215.  Fig.  135  represents  a  very  beautiful  result,  obtained  on  a  silver  plate  exposed 
first  to  the  vapour  of  iodine,  and  then  for  a  short  time  to  the  vapour  of  chloride  of 
iodine.  In  this  case  the  point  of  maximum  falls  nearer  the  line  G.  The  time  of  ex- 
posure was  ten  minutes.  The  fixed  lines,  which  were  discovered  by  M.  Becquerel  and 
myself  beyond  the  visible  spectrum,  are  here  crowded  down  into  so  small  a  space  that 
the  individual  groups  are  all  found  together,  so  that  they  would  scarcely  be  recognised. 

216.  Fig.  136  gives  a  very  perfect  result  which  I  obtained  on  a  plate  exposed  first 
to  the  vapour  of  iodine,  then  to  bromine,  and  then  to  chloride  of  iodine;  the  point  of 
maximum  falls  at  0-00001538,  as  in  Jig.  134  ;  the  time  of  exposure  was  one  hour. 
The  decomposition  extended  on  one  side  beyond  E,  in  the  green  space,  to  the  point 
0-00002007  ;  and  beyond  the  violet  space  to  the  point  0-00001257.  As  in  Jig.  134, 
we  here  again  see  the  point  of  maximum  is  not  in  the  middle  of  the  image,  but  is  to- 
wards its  more  refrangible  extremity. 

217.  As  respects  this  spectrum  (Jig.  136),  although  it  extends  to  the  yellow  space,  the 
line  E  is  not  represented  in  it.  I  may  remark  of  these  spectra,  as  was  formerly  re- 
marked (Ap.,  746-747)  in  the  case  of  the  prismatic,  that  the  lines  D  and  E  do  not  ap- 


DECOMPOSITION  OF  CARBONIC  ACID  IN  THE  YELLOW  REGION.  fjj 

pear.  M.  Becquerel,  as  appears  from  the  Scientific  Memoirs,  vol.  iii.,  pi.  ix.,Jig.  2, 
has  been  more  fortunate.  Although,  in  the  course  of  a  great  many  experiments,  I  have 
often  obtained  very  beautiful  results,  in  which  the  lines  in  the  indigo  and  violet  spaces 
were  given  in  great  numbers,  and  many  of  them  of  that  degree  of  minuteness  as  to  re- 
quire a  lens  to  show  them  distinctly,  it  has  never  yet  happened  to  me  to  see  the  fixed 
lines  D  and  E  reproduced  on  a  sensitive  surface  of  any  kind. 

2]  8.  It  may  be  remarked,  in  conclusion,  that  just  as  we  see  the  colorific  rays  symmet- 
rically arranged  in  these  interference  spectra,  and  the  chemical  results  expressed  by  the 
decomposition  of  chloride  and  bromide  of  silver  crowded  into  a  narrow  space,  compared 
with  what  takes  place  in  the  prismatic  spectrum,  so  must  the  same  thing  hold  for  the 
rays  of  heat,  the  apparent  distribution  of  which  must  be  totally  altered. 


CHAPTER  VI. 

EXPERIMENTS  PROVING  THAT  IT  IS  IN  THE  YELLOW  REGION  OF  THE  SPECTRUM  THAT  THE 
REDUCTION  OF  CARBONIC  ACID  BY  THE  LEAVES  OF  PLANTS  TAKES  PLACE. 

Contents  :  Several  Lnponderahle  Triyiciples  in  the  Sunbeam. — Sennehier  s  Experi- 
ments to  determine  to  which  of  these  the  Effect  is  due. — Experiments  of  Morren  and 
Dauheny. — Defects  of  the  Mode  of  operating  with  Absorhent  Media  and  Glasses. 

Decomposition  of  Carbo?iic  Acid  in  the  Pris?natic  Spectrum. — Process  of  coiiductiug 
the  Experiment. — It  is  in  the  Yelloiv  Region  that  the  Decomposition  takes  place. — ■ 
No  Gas  is  evolved  in  the  Violet. 

219.  The  elementary  views  of  the  constitution  and  nature  of  the  spectrum  which 
we  have  given  enable  us  now  to  return  to  the  physiological  problem  under  discussion. 
From  Chapter  II.  we  see  that  the  process  of  digestion  of  plants,  so  far  as  we  have  yet 
examined  it,  may  be  separated  into  three  different  chemical  actions,  1st.  The  absorp- 
tion of  carbonic  acid  and  water.  2d.  The  evolution  of  a  mixture  of  oxygen  and  nitro- 
gen.   3d.  The  retention  by  the  plant  of  carbon,  hydrogen,  oxygen,  and  nitrogen. 

220.  There  has,  therefore,  been  a  decomposition  of  carbonic  acid,  of  water,  and 
probably  of  some  nitrogenized  compound. 

221.  But  these  extraordinary  decompositions  have  been  produced  by  the  agency  of 
the  sun.  The  most  energetic  reducing  agents  which  we  know  are  required  in  the 
hands  of  chemists  to  produce  some  of  these  effects.  When  made  red  hot,  the  vapour 
of  phosphorus  and  also  potassium  will  accomplish  the  deoxydation  of  carbonic  acid. 
Plants  can  do  the  same  at  50°  Fah.  very  readily. 

222.  If  water  be  exposed  to  the  light  in  glass  vessels,  common  observation  assures 
us  that  it  never  undergoes  decomposition.  In  the  same  manner  carbonic  acid  in  a  jar 
remains  without  exhibiting  any  change.  We  shall  soon  discover  why.  under  these 
circumstances,  the  powerful  reducing  agency  of  light  is  not  called  into  action. 


62 


SEVERAL  IMPONDERABLE  PRINCIPLES  IN  THE  SUNBEAM. 


223.  We  have  seen  that  the  appearance  of  green  matter  in  water,  and  the  produc- 
tion of  chloroph}!  in  leaves,  are  the  same  phenomenon.  In  the  largest  trees  all  the 
solid  matter  found  in  their  branches,  stems,  bark,  and  various  other  parts,  was  originally 
fabricated  in  the  leaves,  for  it  was  in  them  that  carbonic  acid  was  decomposed  and  its 
solid  material  fixed.  Once  thus  introduced  into  the  interior  of  the  vegetable  system, 
it  passes  through  a  multitude  of  changes;  from  the  descending  sap  the  different  com- 
pounds are  removed,  and  lodged  in  their  proper  places.  It  is  in  this  way  that  the 
starch,  sugar,  gum,  woody  fibre,  fibrine,  albumen,  caseine,  essential  oils,  resins,  &c.,  are 
all  produced.  First  one  and  then  another  change  is  impressed  on  the  descending  juice, 
and  first  one  and  then  another  special  compound  is  removed  from  it ;  but  the  point 
from  which  all  this  cycle  of  changes  begins  may  be  traced  back  into  the  leaf,  to  the 
decomposition  of  carbonic  acid  and  production  of  chlorophyl,  under  the  influence  of 
light. 

224.  A  ray  of  light,  as  is  well  known  (137),  contains,  under  ordinary  circumstances, 
several  distinct  principles.  1st.  A  principle  which  impresses  the  organ  of  vision  with 
a  specific  sensation,  and  which  is  therefore  spoken  of  explicitly  as  light  ;  of  this  there 
are  several  modifications :  one  which  produces  in  our  eyes  the  sensation  of  a  red  col- 
our;  another,  a  yellow;  a  third,  a  blue;  and  these,  conjoined  in  proper  proportions,  give 
rise  to  a  white.  2d.  A  principle  of  radiant  heat,  the  well-known  characters  of  which 
are  the  power  of  producing  expansion  in  bodies,  of  communicating  a  feeling  of  warmth; 
in  this  the  phenomena  of  coloration  have  been  traced  as  in  the  former  case.  3d.  A 
principle  of  chemical  force,  which  seems  to  be  intimately  associated  with  light,  but  dif- 
fers from  Ji  It  principle  in  wanting  the  power  of  affecting  our  organs  of  vision.  From 
this  intimate  association  with  light,  I  have  suggested  for  this  principle  the  name  of  Ti- 
THONic  RAYS,  in  allusiou  to  the  fable  of  Tithonus  and  Aurora.  In  this,  as  in  the  for- 
mer, the  quality  of  coloration  has  already  been  traced.  4th.  A  principle  of  phospho- 
rescence, which  has  the  distinctive  character  of  causing  certain  substances,  when  sub- 
mitted to  its  influen(;e,  to  shine  for  a  short  time  after  with  a  transient  light ;  this  prin- 
ciple also  is  invisible  to  the  eye,  but  differs  from  the  tithonic  rays  in  the  fact  that,  un- 
der certain  circumstances,  transparent  glass  is  opaque  to  it. 

225.  Of  these  constituents,  visible  and  invisible,  in  the  solar  beam,  which  is  it  that 
has  charge  of  that  digestive  function  of  plants  which  we  are  now  considering  t  Is  it 
the  light  or  heat,  the  tithonic  or  the  phosphorescent  ray  1 

226.  Several  years  ago,  experiments  were  made  by  Sennebier  for  the  purpose  ef  de- 
termining this  question  ;  his  results  seemed  to  show  that  the  violet  rays  contained  the 
active  force.  More  recently,  in  France,  M.  Morren  commenced  an  inquiry  of  a  sim- 
ilar kind,  and  came  to  the  conclusion  that  the  luminous  rays,  and  more  especially  the 
yellow,  were  the  cause  of  the  phenomenon.  In  1836,  Dr.  Daubeny  published,  in  the 
Transactions  of  the  Royal  Society,  a  very  extensive  series  of  experiments,  the  result 
of  which  went  to  show  that  the  leaves  of  plants  decomposed  carbonic  acid,  and  turned 
green  most  rapidly  in  the  yellow  ray,  the  other  colours,  orange,  green,  red,  blue,  indigo, 
and  violet,  producing  the  same  effect,  more  and  more  slowly  in  proportion  as  their  illu- 
minating power  was  less.    The  plan  which  was  followed  by  these  different  chemists 


EXPERIMENTS  OF  SENNEBIER,  MOUUEN,  AND  DAUBENY.  53 

was  to  cause  the  plants  to  grow  or  to  carry  forward  their  decomposing  action  under 
the  influence  of  light  which  has  passed  through  glass  stained  with  the  different  colours 
Thus,  it'  young  plants  are  made  to  grow  beneath  a  shade  of  lemon-yellow  glass,  in  a 
few  days  it  is  seen  that  such  liglit  does  not  exert  a  destructive  agency,  but  they  grow 
vigorously  ;  and  if,  simultaneously,  comparative  trials  are  made  with  other  shades  of 
different  colours,  such  as  red,  blue,  green,  &c.,  an  estimate  may  be  obtained  of  the  com- 
parative effects  of  rays  of  these  different  colours. 

227.  But  it  is  well  known  that  lights  thus  produced  are  not  monochromatic,  but 
contain  a  variety  of  different  coloured  rays  ;  for  when  the  ray  which  passes  through 
them  is  examined  by  a  prism,  it  is  always  dispersed  into  a  variety  of  different  colours. 
Thus,  the  light  which  passes  through  the  ordinary  variety  of  blue  glass,  analyzed  in  this 
way,  is  found  to  contain  red,  yellow,  and  an  abundance  of  indigo  and  violet,  although  it 
looks  of  a  pure  blue  tint.  The  blue  light  which  passes  it  is  therefore  not  a  simple,  but 
a  composite  blue,  made  up  of  many  other  colours,  the  blue  predominating.  And  as 
this  observation  applies  to  almost  all  sorts  of  coloured  media,  their  colours  being  com- 
pound and  not  simple,  it  is  obvious  that  when  we  make  experiments  with  them  we 
shall  be  liable  to  be  led  into  error,  unless  we  determine  what  rays  they  actually  trans- 
mit. Accordingly,  Dr.  Daubeny  determined  this  point  in  the  case  of  each  glass  and 
coloured  medimn  that  he  used. 

228.  About  the  same  time  that  these  experiments  were  made  in  England,  I  com- 
menced some  of  the  -same  kind  in  the  south  of  Virginia  (Ap.,  4]  3,  497,  506,  515-517), 
where,  during  the  summer  season,  the  light  is  extremely  brilliant.  These  served  to 
show  that  under  a  yellow  solution,  such  as  the  bichromate  of  potash  affords  when  dis- 
solved in  water,  the  leaves  of  plants  turned  green,  and  the  decomposition  of  carbonic 
acid  gas  was  effected. 

229.  Soon  after  this  the  British  Association  for  the  Advancement  of  Science,  in 
consequence  of  some  results  which  had  been  obtained  in  England,  which  seemed  to 
countenance  the  opinion  of  Sennebier,  that  the  violet  rays  direct  the  function  of  di- 
gestion, appointed  a  connnittee  to  examine  this  matter  at  their  expense.  A  number  of 
experiments  were  consequently  made,  very  much  in  the  same  manner  as  those  which 
had  been  previously  conducted  by  Dr.  Daubeny  ;  these  seemed,  however,  to  be  at  va- 
riance with  his  conclusions,  and  showed,  that  while  the  violet  tithonic  ray  is  the  ac- 
tive agent  which  controls  the  process,  the  yellow  ray  of  light  is  exceedingly  injurious. 

230.  But  it  is  now  known  that  these  results,  like  those  of  Sennebier,  are  erroneous, 
and  that,  as  was  first  discovered  by  Morren  in  France,  and  by  Daubeny  in  England, 
it  is  yellow  light  which  directs  the  digestion  of  plants ;  that  leaves  can  be  greened, 
and  oxygen  given  off,  and  carbon  fixed  by  light  that  has  passed  through  a  solution  of 
bichromate  of  potash,  or  through  yellow  glass  (Ap.,  784-792). 

231.  The  proper  mode  of  conducting  these  experiments  is  to  employ  the  solar  spec- 
trum itself,  a  process  which,  it  will  be  seen  (Ap.,  Ch.  XV.),  was  first  resorted  to  in 
New-York.  By  some  appropriate  optical  mechanism,  a  ray  of  the  sun  is  transmitted 
into  a  dark  room  through  a  circular  hole  in  one  of  the  shutters,  and  kept  motionless 
lor  several  hours,  although  the  sun  may  have  apparently  moved  a  considerable  distance 


64 


DECOMPOSITION  OF  CARHOMIC  ACID  IN  THE  PRISMATIC  SPECTRUM. 


over  the  sky.  This  ray  {a  b,Jig.  122)  is  to  he  intercepted  hy  a  glass  prism,  c,  which 
disperses  it  into  its  different  coloured  beams.  And  now,  having  provided  a  set  of  glass 
tubes  filled  with  spring  water,  or,  rather,  water  holding  carbonic  acid  gas  in  solution, 
and  in  each  placed  the  same  number  of  leaves  of  grass  or  of  some  other  plant,  so  that 
each  tube  may  be  as  nearly  like  all  the  others  as  may  be,  these  tubes,  inserted  in  a  small 
pneumatic  trough,  c  d,  are  to  be  set  in  the  spectrum  of  light,  in  its  different  coloured 
spaces,  one  in  the  red,  one  in  the  orange,  one  in  the  yellow,  &c.  Care  also  should  be 
taken  to  exclude  all  extraneous  light,  so  that  that  which  causes  any  action  among  the 
leaves  may  be  derived  from  the  ray  which  comes  in  through  the  shutter  only.  Very 
soon,  if  the  sky  is  clear  and  the  sun  brilliant,  the  phenomenon  begins.  In  the  tube 
which  is  in  the  most  luminous  part  of  the  yellow  ray  small  bubbles  are  evolved,  and 
these  rising  to  the  top  of  that  tube,  there  collect,  so  that  after  the  lapse  of  a  few  hours 
a  sufficient  quantity  may  be  gathered  for  measurement  and  analysis.  The  tubes  that 
are  in  the  orange  and  green  lights  simultaneously  go  into  action,  and  when  the  sky 
possesses  an  intense  brilliancy,  they  will  even  approach  in  the  rapidity  of  their  action 
to  the  maximum  yellow.  A  few  bubbles  also  make  their  appearance  in  the  blue,  but 
after  an  exposure  of  many  hours,  if  scrupulous  care  is  taken  to  shut  out  all  extraneous 
light,  no  action  whatever  is  perceptible  in  the  extreme  violet,  where  Sennebier  supposed 
the  decomposing  force  to  be  situated.  From  these  things  we  therefore  gather,  that  it 
is  in  the  yellow  light  that  the  power  controlling  the  function  of  digestion  of  plants  is 
to  be  found,  the  other  coloured  beams,  orange,  green,  red,  blue,  &c.,  following  in  the 
order  of  their  illuminating  power. 

232.  This  prismatic  experiment  is  one  of  the  most  beautiful  objects  which  organic 
chemistry  can  offer,  carried  on  in  a  chamber  which  would  be  totally  dark  were  it 
not  for  the  intensely  coloured  curves  which  are  cast  upon  the  walls  by  reflexion  from 
the  tubes,  curves  which  often  are  many  yards  in  length,  indicating  by  their  gaudy  tints 
and  brilliancy  the  intensity  of  the  sun's  light.  The  tubes  and  the  vegetable  leaves  glow 
with  the  colours  in  which  they  are  immersed.  Meantime,  the  most  interesting  phe- 
nomenon which  can  be  witnessed  is  silently  going  forward ;  dead  and  inanimate  matter 
is,  under  the  influence  of  the  plastic  beam,  putting  on  the  form  of  organization  and 
life.  Oxygen  and  nitrogen  gases  are  exhaling,  chlorophyl,  and  gum,  and  sugar,  fib- 
rine,  and  albumen  are  coming  into  existence.  These  are  compounds  which,  under 
ordinary  circumstances,  are  destined  to  be  used  as  the  food,  and  form  part  of  the  bodies 
of  animals.  For  it  is  from  atmospheric  air,  as  organic  chemistry  shows,  that  plants 
spring,  condensed  out  of  it,  as  it  were,  by  the  agency  of  the  solar  beam;  to  the  same 
source  also  it  is  that  gradually  during  life,  and  totally  after  death,  the  parts  of  animals 
hasten  to  return. 

233.  A  little  reflection  shows  the  great  advantages  which  this  mode  of  experiment- 
ing possesses.  In  the  white  light  of  the  solar  beam  there  is  a  fixed  proportion  of  each 
of  the  component  colours,  and  when  such  a  beam  is  dispersed  by  prismatic  action,  and 
is  simultaneously  received  upon  vegetable  leaves,  we,  in  effect,  measure  out  to  them 
similar  quantities  of  the  different  coloured  rays,  and  observe  the  resulting  action.  When 
pieces  of  glass  are  used,  as  in  the  former  experiment  (226),  a  great  deal  depends  on  their 


OBJECTIONS  TO  THE  USE  OF  COLOURED  GLASSES.  55 

thickness.  It  is  easy  to  conceive,  that  aUhough  in  the  solar  spectrum  the  yellow  ray 
is  vastly  more  luminous  than  the  blue,  if  glasses  or  coloured  media  were  used  in  experi- 
ments of  any  kind,  results  of  precisely  an  opposite  nature  might  be  obtained.  We  can 
imagine  a  piece  of  yellow  glass  so  thick  as  actually  to  transmit  less  light  than  a  thinner 
piece  of  the  blue.  With  glasses  and  absorbent  media,  therefore,  not  only  must  the  na- 
ture of  transmitted  light  be  discovered,  but  thickness  must  also  be  taken  into  account. 

234.  Suppose,  as  an  illustration,  plants  were  made  to  grow,  or  to  decompose  car- 
bonic acid,  under  a  thin  piece  of  blue  cobalt  glass,  such  as  that  of  which  finger-glasses  are 
made,  and  the  result  compared  with  what  was  occurring  with  another  set  of  plants  in 
action  under  a  thick  or  deep  yellow  glass.  It  might  turn  out  that  the  former  would 
vegetate  more  vigorously,  and  fix  more  carbon,  and  produce  more  chlorophyl  than  the 
latter,  because  more  light  actually  fell  upon  them,  though  the  medium  through  which  it 
came  was  blue.  We  must  not  forget  that  the  mode  of  action  in  producing  chemical 
decomposition,  and  the  mode  of  action  in  producing  vision  through  such  an  optical 
contrivance  as  an  eye,  proceed  upon  different  principles.  In  the  action  of  the  eye 
time  does  not  enter  as  an  element  (Ap.,  536).  In  the  decompositions  produced  by  ra- 
diant matter,  or  by  light,  it  does.  A  faintly  luminous  object  does  not  become  brighter 
and  brighter  as  we  continue  to  look  steadfastly  at  it ;  it  has  assumed  its  maximum  of 
brilliancy  at  the  first  glance.  But  a  faintly  luminous  beam,  falling  upon  leaves  of 
plants,  or  upon  any  changeable  compound,  continues  to  produce  an  increasing  effect 
as  time  passes  on.  In  our  experiment,  more  and  more  carbonic  acid  is  decomposed, 
and  more  and  more  oxygen  set  free  as  the  time  is  prolonged.  And  so  when  absorbent 
media,  as  stained  glass,  are  used,  the  final  effect  is  dependant  on  the  total  amount  of 
light  that  has  been  furnished ;  and  hence  the  turbidity,  or  thickness,  or  partial  opacity 
of  those  media  must  be  taken  into  account,  as  much  as  their  colour-giving  relations. 
In  the  prismatic  experiment  this  source  of  disturbance  does  not  occur. 


CHAPTER  VII. 

ON    THE   VARIOUS   IMPONDERABLE    AGENTS    EXISTING   IN   THE    DIFFERENT    REGIONS  OF 

THE  SPECTRUM. 

Contents  :  Different  Agents  existing  in  the  Spectrum. — Description  of  the  Tithonic 
Rays. —  Their  Name. — Physical  Independence  of  Heat. — Of  the  Chemical  Rays. — 
Their  Constant  Association  with  Light. — Detithonizing  Action  of  Yelloiv  Solutions. 
— Argument  for  their  Independence. — Other  Invisible  Principles  in  the  Sunbeam, 
such  as  the  Phosphoric  Rays. — Examination  of  the  Theory  of  M.  Becquerel. 

235.  From  the  preceding  chapter,  we  have  arrived  at  the  conclusion  that  the  impon- 
derable principle  which  directs  the  digestion  of  plants  is  found  at  a  maximum  in  the 
yellow  space  of  the  spectrum,  or  accompanies  yellow  light. 

236.  But  it  has  been  stated  (224)  that  there  are  no  less  than  three  other  principles 

I 


66 


DESCRIPTION  OF  THE  TITHONIC  RAYS. 


coexisting  in  solar  light,  tithonic,  calorific,  and  prosphorescent  rays ;  for  the  two  for- 
mer the  phenomenon  of  coloration  has  been  distinctly  traced,  as  will  appear  in  a  sub- 
sequent chapter ;  for  the  latter,  analogy  would  lead  us  to  suppose  that  the  same  thing 
holds. 

237.  In  yellow  light,  such  as  can  effect  the  decomposition  of  carbonic  acid,  a  vari- 
ety of  other  principles  exist — yellow  tithonic  rays,  yellow  calorific  rays,  and  perhaps 
yellow  phosphorescent  rays.  Prismatic  experiments  (Ap.,  790)  can  only  indicate  the 
refrangibility  of  the  ray  which  produces  a  given  effect.  Experiments  by  absorbent  me- 
dia are  finally  required  to  point  out  its  exact  nature. 

238.  In  order  that  the  reader  may  possess  clear  views  of  the  evidence  which  is  to 
be  brought  forward,  showing  that  it  is  yellow  light,  and  not  tithonic,  calorific,  or  phos- 
phorescent yellow  rays  which  superintend  the  formation  of  organic  molecules  out  of 
inorganic  matter,  it  is  necessary  to  set  forth  in  a  more  prominent  way  the  distinctive 
characteristics  which  appertain  to  each  of  these  principles. 

239.  Of  the  calorific  rays,  or  rays  of  heat,  nothing,  however,  need  here  be  said.  Their 
physical  independence  of  light,  and  separate  existence,  were  completely  established  by 
Melloni  (170),  who  also  discovered  in  them  modifications  analogous  to  coloration. 

240.  The  existence  of  the  tithonic  rays  was  first  ascertained  during  the  last  century. 
They  passed  in  the  books  under  the  name  of  chemical  rays — a  name  still  continued  by 
many  chemists.  As  a  multitude  of  phenomena  with  which  they  are  connected  became 
known,  it  has  been  found  absolutely  essential  to  give  to  them  a  specific  name,  such  as 
can  be  moulded  conveniently  for  the  purposes  of  science,  and  combine  readily  to  form 
those  different  words  which  are  necessary  to  give  names  to  instruments  in  which  they 
are  involved,  or  to  phenomena  with  which  they  are  connected — a  name  which,  how- 
ever, involves  no  hypothetical  idea  of  their  nature  or  action. 

241.  The  most  striking  fact  in  connexion  with  these  rays  is  their  constant  associa- 
tion with  light.  Though  the  two  principles  are  separable  from  each  other  by  artificial 
processes,  yet,  under  natural  circumstances,  they  always  occur  together.  In  sunlight, 
in  gas-flames,  from  candles,  and  even  in  the  moonbeams,  in  which  no  heat  is  found,  the 
chemical  and  luminous  rays  exist  together.  They  undergo  reflexion,  refraction,  polar- 
ization, and  interference,  apparently  under  the  very  same  laws.  But  while  light  can 
affect  the  eye,  and  bring  us  into  relation  with  all  the  phenomena  of  the  material  world, 
these  rays  fail  to  affect  our  organs  of  vision — they  are  invisible. 

242.  In  view  of  this  constant  association  of  light  and  the  chemical  rays,  and  to  ex- 
press the  idea  of  their  near  alliance  to  each  other,  and  as  the  wants  of  science  imper- 
atively called  for  a  specific  designation,  the  name  of  tithonic  rays  has  been  suggested  for 
them.  It  is  drawn  from  the  fable  of  Aurora  and  Tithonus.  This  name  forms  compound 
words  with  facility.  It  does  not  involve  us  in  any  speculative  considerations,  but  merely 
points  out  the  fact  that  the  principle  to  which  it  is  given  is  almost  always  associated  with 
light.  As  has  been  just  stated,  all  the  mechanical  laws  which  regulate  the  reflexion, 
transmission,  polarization,  and  interference  of  light,  obtain  also  for  this. 

243.  The  mathematical  theory  of  light  is  based  upon  the  postulate  of  undulations 
taking  place  in  an  ethereal  medium.    This  theory,  like  the  theory  of  universal  gravi- 


THEIR  PHYSICAL  INDEPENDENCE.  gy 

tation,  possesses  an  abundant  internal  evidence  of  truth.  In  our  times  extensive  and 
important  applications  have  been  made  of  it,  so  that  it  now  includes  an  explanation  of 
all  the  phenomena  of  reflexion,  refraction,  polarization,  double  refraction,  interference. 
As  yet,  it  has  furnished  no  cl  ar  account  of  the  phenomena  of  absorption,  the  very  phe- 
nomena which  are  at  the  basis  of  all  physiological  and  chemical  facts  in  their  relations 
to  luminous  agency.  As  we  shall  presently  prove,  the  decomposition  of  carbonic  acid 
and  the  function  of  digestion  in  plants  depend  on  the  absorption  of  light.  Nor  is 
there  yet  included  in  it  any  representation  of  the  various  phenomena  of  heat.  In  the 
course  of  years,  these  things  will  probably  be  added,  and  one  great  generalization  em- 
brace all  the  phenomena  of  the  imponderable  agents.  But  the  purposes  of  science  re- 
quire, until  that  event  takes  place,  that  we  should  continue  to  speak  of  the  various 
imponderable  principles  as  though  they  were  diflerent  agents,  and  treat  them  as  though 
they  were  separate  existences. 

244.  The  first  decisive  evidence  brought  forward  to  establish  the  physical  independ- 
ence of  the  heat  and  light  of  the  sunbeam,  as  has  been  stated  (165),  was  derived  from 
the  experiments  of  Sir  W.  Herschel,  the  illustrious  astronomer.  He  foun  1  that,  when 
a  beam  of  light  is  dispersed  by  a  prism,  and  the  resulting  spectrum  examined,  by  placing 
thermometers  in  its  different  coloured  spaces,  the  most  luminous  rays  are  not  the  hot- 
test, the  maximum  temperature  occurring,  not  in  the  yellow,  but  in  the  red  ray ;  and  even 
out  of  the  red  ray,  and  where  the  eye  could  detect  no  light  whatever,  heat  was  present, 
for  the  thermometer  there  rose  rapidly.  Starting  from  these  experiments,  Melloni  added 
farther  proof  by  showing  that  transparency  for  light  is  not  necessarily  transparency  for 
heat ;  that  there  are  certain  media  more  or  less  opaque  to  one  of  these  imponderables, 
and  more  or  less  transparent  to  the  other,  and  that,  b}  appropriate  combinations,  media 
can  be  obtained  which  will  allow  light  to  pass  them  with  very  little  diminution  of  its 
intensity,  but  which  stop  the  heat  almost  entirely. 

245.  In  the  dark  rays  which  come  from  a  vessel  of  hot  water,  we  have  radiant  heat 
without  light ;  in  the  moonbeams  we  have,  on  the  contrary,  light  without  heat. 

246.  By  a  series  of  experiments  of  a  similar  kind,  the  physical  independence  of  the 
tithonic  rays  has  also  been  established.  During  the  last  century,  it  was  observed  by 
SciiE  LE,  that  these  rays  occur  al)undantly  beyond  the  violet  extremity  of  the  spectrum, 
where  the  eye  can  discern  no  trace  of  light,  an  observation  essentially  of  the  same  kind 
as  that  made  by  Sir  W.  Herschel  for  radiant  heat.  In  consequence  of  this  discovery, 
the  occurrence  of  invisible  rays  was  at  once  assumed,  and,  without  any  inquiry  as  to 
their  nature,  their  existence  became  an  admitted  fact  in  science. 

247.  It  does  not  appear,  however,  that  any  clear  views  were  entertained  as  to  the 
precise  character  of  these  dark  rays.  Writers  on  optics  spoke  of  them  indifferently 
under  the  name  of  violet  rays,  chemical  rays,  deoxydizing  rays,  and  invisible  rays.  Of 
all  the  benefits  which  can  be  conferred  on  an  infant  science,  those  arising  from  estab- 
lishing clear,  bold,  prominent,  decisive  views  of  its  fundamental  agencies  and  their  ac- 
tions are  by  far  the  most  valuable,  for  they  fasten  the  attention  forcibly.  In  all  pro- 
gressive sciences,  each  epoch  of  evolution,  for  sciences  advance  forward  by  starts,  is 
traceable  to  the  announcement  of  some  clear  and  vivid  idea. 


68 


DETITHONIZING  ACTION  OF  YELLOW  SOLUTIONS. 


248.  As  we  have  said,  the  existence  of  these  dark  rays  beyond  the  violet  end  of  the 
spectrum  was  estabUshed  during  the  last  century.  For,  when  a  solar  spectrum  is  made 
to  fall  on  a  piece  of  paper  covered  over  with  white  chloride  of  silver,  that  compound 
turns  black,  the  colour  changing  where  blue,  indigo,  and  violet  rays  fall ;  the  change 
also  extending  over  those  spaces  which  are  outside  of  the  violet  ray,  and  where  there 
is  no  light.  In  1836,  by  using  bromide  of  silver  instead  of  chloride,  I  found  that  these 
deoxydizing  rays  extended  out  of  the  blue  light  down  towards  the  yellow  (Ap.,  445)  ; 
these  experiments  were  published  in  1837. 

249.  From  a  variety  of  evidence,  obtained  about  the  same  time,  it  appeared  that 
when  absorptive  media  were  used,  the  rays  of  light  could  be  deprived  of  much  of  their 
chemical  power.  Thus,  on  passing  a  ray  through  a  solution  of  bichromate  of  potash 
(Ap.,  410),  it  became  almost  inactive,  refusing  to  produce  any  change  on  sensitive  paper; 
a  similar  result  following  on  passing  it  through  Dalton's  solution  of  the  quadrosulphu- 
ret  of  lime,  and  a  variety  of  other  media  (Ap.,  410-509).  At  that  early  period  in  pho- 
tographical  investigations,  when  as  yet  no  other  chemist  was  engaged  in  these  pursuits, 
two  important  facts  were  published :  1st.  The  physical  independence  over  a  great  part 
of  the  spectrum  of  light  and  the  tithonic  rays  (Ap.,  386).  2d.  That  the  tithonic  rays 
exhibit  undoubted  tokens  of  modifications  answering  to  colours  in  light  (Ap.,  384,  433, 
506).  This  is  the  foundation  of  the  doctrine  now  more  completely  unfolded  in  this 
work,  under  the  designation  of  "  The  Theory  of  Ideal  Coloration  of  the  Tithonic  Rays." 

250.  From  their  constant  association  with  light,  it  is  difficult  to  give  that  clear  evi- 
dence of  the  physical  independence  of  the  tithonic  rays  which  may  be  given  in  the 
case  of  heat.  Much  weight  may,  nevertheless,  be  laid  upon  the  circumstance  that,  by 
prismatic  action,  these  rays  can  be  proved  to  exist  beyond  the  extreme  violet  region,  and 
even  beyond  that  ray  which  Sir  J.  Herschel  designates  lavender.  Evidence  of  the 
same  kind  is  derived  from  the  partial  action  of  absorbent  solutions  and  media,  which 
act  differently  on  each  of  these  classes  of  rays.  From  observations  of  this  kind  tables 
have  been  formed,  with  a  view  of  proving  this  point  (Ap.,  673).  In  the  Transactions 
of  the  Royal  Society,  Sir  John  Herschel  has  given  some  of  the  same  sort  (P7a7. 
Trans.,  1840).    My  results  were  published  in  1842. 

251.  At  one  time  much  stress  was  laid  on  the  power  which  certain  media,  such  as 
bichromate  of  potash  and  quadrosulphuret  of  lime,  possess  in  depriving  a  ray  of  its 
chemical  force.  This  argument  is  still  resorted  to  by  inaccurate  writers,  as  affording 
a  very  popular  and  palpable  proof  that  light  and  the  tithonic  rays  are  different  ex- 
istences. "  If,"  it  is  observed,  "  a  ray  of  light  is  passed  through  a  solution  of  quadro- 
sulphuret of  lime,  and  is  then  received  on  paper  covered  with  chloride  of  silver,  it  is 
found  that  the  light  alone  has  gone  through,  and  the  chemical  rays  have  been  absorbed. 
The  one,  therefore,  exists  independent  of  the  other."  When  I  first  observed  the  power 
of  these  yellow  solutions,  this  was  the  conclusion  I  came  to  (Ap.,  410,  511).  But  the 
theory  of  ideal  coloration,  presently  to  be  explained,  deprives  this  argument  of  much  of 
its  force ;  for  the  rays  that  are  transmitted  by  such  a  solution  are  yellow  rays  of  light, 
and  yellow  tithonic  rays ;  in  these  the  chloride  of  silver  does  not  change,  its  decompo- 
sition being  brought  about  by  the  blue  tithonic  rays  which  have  been  absorbed  along 


OTHER  INVISIBLE  PRINCIPLES  IN  THE  SUNBEAM. 


69 


with  the  bhie  hght.  Tithonic  rays  are  present  in  a  beam  which  has  passed  through 
quadrosulphuret  of  hme,  but  chloride  of  silver  cannot  detect  them. 

252.  An  accurate  examination  of  the  quantity  of  hght  and  the  quantity  of  tithonic 
rays  passing  through  given  solutions  has,  however,  restored  this  argument  to  its  pristine 
force.  Thus,  it  is  proved,  that  of  different  media  which  have  been  tried,  some  trans- 
mit more  of  the  luminous  and  some  more  of  the  tithonic  rays  (Ap.,  Cn.  XVIII.)  ;  that 
the  rate  of  transparency  for  one  is  often  totally  different  from  the  rate  of  transparency 
for  the  other ;  and  although  we  have  not  yet  been  so  fortunate  as  to  discover  any  given 
medium  which  is  opaque  to  one  of  these  principles  and  transparent  to  the  other,  as  has 
been  done  in  the  case  of  light,  their  physical  independence  is  just  as  certain  as  though 
such  a  medium  was  known. 

253.  After  commenting  in  the  preceding  paragraph  on  the  weak  point  in  the 
argument  drawn  from  absorbent  media,  it  ought  not  to  be  left  without  showing  how 
experiments  of  that  class  may  be  rigorously  used  as  proofs  of  the  correctness  of  the 
views  given  in  1837.  It  only  requires  that  the  sensitive  paper  or  surface  used  should 
be  of  such  a  character  as  to  be  affected  by  all  the  tithonic  rays  in  the  spectrum,  irre- 
spective of  their  order  of  refrangibility  or  ideal  coloration.  Such  a  substance  is  the 
bromide  of  silver.  Sir  J.  Herschel  has  used  it  with  a  similar  object  in  view  (P/tii. 
Trans.,  1840,  p.  38).  Suppose,  therefore,  we  allow  a  solar  spec.trum  to  fall  upon  such 
a  changeable  surface,  after  having  passed  through  a  yellow  absorbent  medium,  as  the 
bichromate  of  potassa.  The  eye  at  once  informs  us  that  very  little  of  the  yellow  and 
orange  light  is  lost,  but  we  must  keep  the  surface  for  a  long  time  exposed  before  the 
yellow  and  orange  tithonic  rays  will  have  produced  such  a  change  as  they  would  have 
produced  in  a  few  moments  if  the  bichromate  had  not  intervened.  In  this  case  the 
experiment  is  a  fair  one,  and  the  deduction  it  gives  holds  good,  because  bromide  of 
silver  is  easily  decomposed  by  the  tithonic  yellow  ray.  The  bichromate  of  potash, 
therefore,  transmits  yellow  and  orange  rays  of  light  copiously,  but  it  transmits  the 
corresponding  yellow  and  orange  tithonic  ray  to  a  far  less  extent.  The  two  princi- 
ples are  therefore  distinct. 

254.  That  there  is  nothing  unphilosophical  in  supposing  that  an  invisible  principle 
such  as  that  of  which  we  are  speaking  should  exist  in  solar  light,  is  shown  by  the  anal- 
ogy of  radiant  heat,  a  principle  equally  invisible  to  our  eyes,  but  of  which  the  exist- 
ence is  palpable  enough  to  our  other  organs  of  sense.  In  a  dark  room  we  are  utterly 
unable  to  see  a  vessel  of  hot  water,  but  its  calorific  emanations  are  plain  to  the  hand, 
even  at  a  considerable  distance.  In  like  manner,  this  analogy  is  supported  by  the  re- 
cent discovery  of  Becquerel.  For  a  long  time  it  has  been  known  that  there  are  cer- 
tain bodies,  such  as  calcined  oyster  shells,  which  shine  in  darkness  after  a  brief  expo- 
sure to  the  hght.  A  hundred  years  ago  it  was  discovered  that  the  transient  light  of  an 
electric  spark  is  sufficient  to  awaken  the  dormant  glow  of  these  bodies.  Now  Bec- 
querel has  shown,  that  to  the  rays  which  thus  issue  from  an  electric  spark,  and  cause 
this  wonderful  phenomenon,  glass  is  opaque,  that  light  can  pass  through  glass,  but  the 
phosphorescent  rays  cannot.    They  also  are  invisible  to  the  eye. 

255.  It  will  be  seen,  by  referring  to  Ch.  XII.  of  the  A  p.,  that  there  are  certain  phe- 


70 


EXAMINATION  OF  THE  THEORY  OF  M.  BECQUEREL. 


nomena  which  may  be  explained  on  the  supposition  that  the  invisible  tithonic  rays  cs 
cape  by  radiation  from  bodies  which  have  been  impressed  by  them,  those  bodies  simul- 
taneously reverting  to  their  original  condition.  These  results,  and  similar  ones,  have 
of  late  years  attracted  much  of  the  attention  of  experimental  philosophers,  but  the  in- 
quiries involved  are  beset  by  numerous  difficulties  (Ap.,  708,  &c.).  There  is  not,  how- 
ever, anything  impossible,  or  even  unlikely,  in  this  secondary  radiation.  A  phenome- 
non of  exactly  the  same  kind  is  visible  to  the  eye  in  the  case  of  the  phosphorescent  rays, 
when  the  glow  of  light  by  radiation  escapes  away  from  calcined  oyster  shells  after 
they  have  been  illuminated  by  an  electric  spark. 

256.  Dark,  invisible  rays  thus  exist  in  the  sunlight,  and  carry  on  a  variety  of  func- 
tions, and  control  a  variety  of  phenomena.  Of  solar  principles,  four  different  kinds 
have  been  traced  :  rays  of  light,  of  heat,  tithonic,  and  phosphorescent  rays.  The  two 
former  are  admitted  to  constitute  recognised  imponderable  principles.  What  are  the 
latter  two  ? 

257.  In  the  Philosophical  Magazine  (see  Ap.,  Ch.  XIII.)  I  have  some  years  ago 
brought  forward  the  doctrine,  that  we  are  compelled  to  enlarge  our  catalogue  of  im- 
ponderable principles,  and  include  these  tithonic  rays  in  it.  More  recently  I  have  of- 
fered similar  arguments  in  favour  of  the  phosphorescent  rays  (Ap.,  Ch.  XVIIL). 

258.  This  brings  me  to  offer  some  remarks  on  the  opinion  expressed  by  M.  Bec- 
QUEREL,  that  the  phenomena  now  under  discussion  are  due  to  the  qualities  of  the  re- 
ceiving surfaces,  and  not  to  agents  intrinsically  different,  coexisting  in  the  solar  beam. 
That  the  same  beam  of  light,  falling  on  sulphuret  of  lime,  causes  it  to  phosphoresce  ; 
on  chloride  of  silver,  blackens  it;  on  the  retina,  gives  rise  to  the  phenomena  of  vision 
and  colour ;  on  a  piece  of  black  cloth,  causes  it  to  become  warm.  This  opinion  seems 
to  be  surrounded  with  insurmountable  difficulties,  and,  if  admitted,  would  disturb  some 
of  the  best-established  ti  uths  of  science. 

259.  No  one  can  feel  more  strongly  the  absurdity  of  supposing  that  Nature  has  cre- 
ated between  forty  and  fifty  elementary  ponderable  substances,  all  possessed  of  metallic 
characters,  and  all  so  nearly  alike  that  even  a  chemist  is  often  puzzled  to  distinguish 
them  from  one  another.  No  one,  upon  satisfactory  proof,  would  more  willingly  go 
back  to  the  alchemical  doctrine  in  relation  to  these  matters ;  but  so  long  as  the  evi- 
dence on  the  constitution  of  these  bodies  rests  where  it  does,  the  laws  of  chemistry 
compel  us  to  admit  them  to  be  simple  and  undecompounded.  And,  just  in  the  same  way 
that  I  am  willing  to  admit  the  existence  of  forty  different  simple  metals,  so,  upon  sim- 
ilar evidence,  I  am  free  to  admit  the  existence  of  fifty  different  imponderable  agents,  if 
need  be.  Is  there  anything  which  should  lead  us  to  suppose  that  the  imponderables 
are  constituted  by  Nature  on  a  plan  that  is  elaborately  simple,  and  the  ponderables  on 
one  that  is  elaborately  complex  ?  That  the  former  are  all  modifications  of  one  primor- 
dial ether,  and  the  latter  intrinsically  different  bodies,  more  than  a  quarter  of  a  hun- 
dred of  which  have  been  discovered  during  the  present  century "? 

260.  Before  admitting  the  correctness  of  the  hypothesis  of  M.  Becquerel,  that  the 
agent  itnder  consideration  is  one  and  indivisible,  and  that  all  the  phenomena  we  dis- 
cover are  due  to  the  receiving  surfaces,  and  that  there  are  as  many  spectra  as  there  are 


EXAMINATION  OF  THE  THEORY  OF  M.  BECQUEREL.  71 

substances  in  nature,  each  giving  its  own  manifestations  when  exposed  to  the  sun's  ray, 
we  should  make  inquiries  hke  the  following :  How  is  it  that  a  piece  of  black  cloth  ex- 
posed to  the  moonbeams  does  not  become  warm  ?  How  is  it  that  a  cannister  of  hot 
water  is  not  luminous  to  the  eye  ?  In  the  rays  that  come  from  the  moon,  and  those 
that  are  emitted  by  warm  water,  is  there  no  intrinsic  difference,  or  does  the  phenom- 
enon depend  on  the  receiving  surface  alone  1  What  becomes  of  the  beautiful  exper- 
iments of  Melloni,  on  the  physical  independence  of  light  and  heat,  since  these  are 
mainly  founded  on  the  fact,  that  by  the  use  of  absorbent  media  we  can  separate  one 
from  the  other  ]  How  is  it  that  the  rays  of  an  electric  spark,  passing  through  quartz, 
can  make  the  Bolognian  stone  phosphoresce,  but  passing  through  glass,  equally  trans- 
parent and  equally  colourless,  can  do  no  such  thing  1  The  receiving  surface  is  the 
same  in  both  cases,  and,  as  far  as  human  eyesight  can  discover,  the  light  that  comes 
through  the  glass  is  as  pure  and  unaltered  as  the  light  that  came  through  the  quartz, 
but  the  resuhs  are  diametrically  opposed.  And  is  it  not  more  consonant  to  reason  to 
suppose  that  the  glass  was  opaque,  or  impervious  to  some  agent  existing  in  that  beam, 
which  freely  passed  the  quartz — opaque  to  it,  but  transparent  both  to  light  and  the 
tithonic  rays  1 

261.  We  might  multiply  cases  like  these,  and  give  similar  arguments  from  chemical 
changes  on  sensitive  surfaces;  but  the  instances  already  cited  seem  abundantly  suffi- 
cient to  overturn  the  hypothesis  in  question.  Before  it  can  be  admitted,  it  must  give 
a  reason  why  the  retina  is  not  affected  with  the  sensation  of  vision  when  rays  from  hot 
water  fall  on  the  eye,  why  a  thermometer  will  not  rise  when  placed  in  the  moonshine, 
why  sulphuret  of  lime  or  barytes  will  not  phosphoresce  when  covered  with  a  piece 
of  glass. 

262.  We  are  thus  forced  to  admit  that  rays  of  light,  rays  of  heat,  tithonic  rays, 
phosphoric  rays,  and  probably  many  other  radiant  forms,  have  an  independent  exist- 
ence, and  that  they  can  be  separated,  by  proper  processes,  from  each  other. 

263.  It  must,  however,  be  understood,  that  the  conclusion  here  arrived  at  essentially 
depends  on  the  following  facts  :  1st.  The  constant  visibihty  of  light.  2d.  The  uniform- 
ity of  the  action  which  beat  exhibits  in  expanding  bodies.  If  the  progress  of  science 
should  require  us  to  admit  that  there  can  exist  light  which  impresses  our  eyes  with  a 
sensation  of  darkness,  or  heat  which  can  neither  raise  a  thermometer  nor  produce  the 
sensation  of  warmth,  then  the  force  of  the  foregoing  arguments  will  be  essentially  af- 
fected. Such  extensive  changes  in  the  universal  acceptation  of  words  will,  unques- 
tionably, be  very  slowly  received. 


72 


YELLOW  LIGHT  CONTROLS  THE  DIGESTION  OF  PLANTS. 


CHAPTER  VIII. 

IT  IS  YELLOW  LIGHT  WHICH  CONTROLS  THE  PROCESS  OF  DIGESTION  IN  PLANTS. 

Contents  :  Examination  as  to  which  of  the  Principles  mentioned  in  the  preceding 
Chapter  is  engaged  in  the  Decomposition  of  Carbonic  Acid. — It  is  not  Radiant 
Heat. — Melloni  on  the  Ideal  Coloration  of  Heat. — Analogies  in  the  Case  of  Light. 
— HerscheVs  Results. 

It  is  not  the  Tithonic  Ray. — Maximum  of  Decomposing  Action  for  Carbonic  Acid  and 
Carbonaceous  Compounds,  like  the  Retina,  is  in  the  Yelloio  Ray. — Hence  the  Maxi- 
mum of  Visible  Illumination  coincides  therewith. 

264.  In  the  last  chapter,  and  in  the  Appendix,  Chaps.  XIII.,  XV.,  and  XVIII.,  we 
have  given  proofs  of  the  separate  existence  of  a  number  of  independent  imponder- 
able principles  in  the  solar  beams,  of  which  only  one  produces  a  specific  effect  upon 
the  eye,  the  others  being  wholly  invisible,  and  known  to  us  by  the  chemical  or  mechan- 
ical effects  they  produce. 

265.  When,  therefore,  a  beam  of  light  falls  upon  a  prism,  and  is  decomposed  by  it, 
and  the  resulting  colours  are  received  upon  a  screen  so  as  to  give  rise  to  a  spectrum, 
such  as  that  represented  in  the  frontispiece,  in  each  portion  of  that  spectrum  these  dif- 
ferent imponderable  principles  are  present.  For  example,  in  the  region  between  the 
lines  A  and  C  there  are  red  rays  of  light,  red  rays  of  heat,  and  red  tithonic  rays.  They 
are  mingled  together  there,  fortuitously,  through  the  optical  action  of  the  prism.  Their 
existence  is  perfectly  separate  and  independent,  and  any  one  of  them  may  be  removed 
by  proper  processes,  and  the  others  left. 

266.  Confining  our  attention  now  to  the  yellow  portion  of  the  spectrum,  in  which 
region  the  decomposition  of  carbonic  acid  takes  place,  it  is  obvious,  upon  these  princi- 
ples, that  there  are  coexisting  there  yellow  light,  yellow  heat,  and  yellow  tithonic  rays. 
It  remains  for  us  to  inquire  to  which  of  these  three  principles  the  decomposition  of 
carbonic  acid  and  the  production  of  green  matter  is  due.  The  phosphorescent  rays 
may  be  left  out  of  the  discussion,  though  these,  with  the  other  three  classes,  are  spread 
all  over  the  spectrum. 

267.  First  let  us  ascertain  whether  radiant  heat,  generally,  has  the  quality  of  pro- 
ducing decomposition.  To  rays  coming  from  a  brightly-burning  fire,  I  exposed  some 
vegetable  leaves  in  water  holding  carbonic  acid  gas  in  solution,  and,  to  increase  the 
effect,  converged  the  calorific  rays  by  a  large  metallic  concave  mirror.  That  no  doubt 
might  remain  of  the  incapacity  of  heat  to  produce  the  phenomenon,  the  temperature 
of  the  water,  under  the  influence  of  the  radiant  heat,  was  allowed  to  run  up  to  140° 
Fah.,  a  much  higher  point  than  is  ever  attained  under  natural  circumstances.  But 
neither  at  low  temperatures,  nor  at  these  elevated  ones,  did  any  visible  decomposition 


IDEAL  COLORATION  OF  HEAT.  73 

take  place  ;  showing,  thus,  that  heat  alone  cannot  cause  the  digestion  of  plants.  More- 
over, as  is  well  known  to  chemists,  carbonic  acid  gas  may  be  passed  through  a  tube 
that  is  white  hot  without  giving  the  most  remote  appearances  of  decomposition. 

268.  The  beautiful  experiments  of  JNIelloni  have  proved,  however,  that  rays  of  heat 
emitted  by  bodies  at  different  temperatures  vary  in  their  constitution.  At  a  low  tem- 
perature, such  as  that  of  the  human  hand,  the  caloric  emitted  is  of  a  high  refrangibility, 
and  possesses  invisible  violet  coloration.  As  the  heat  rises,  rays  of  a  lower  refrangi- 
bility are  sent  forth,  so  that  if  we  examine  the  character  of  the  rays  coming  from  a  se- 
ries of  bodies,  the  temperatures  of  which  are  successively  higher  and  higher,  as  the 
hand,  a  vessel  of  boiling  water,  a  red-hot  iron,  a  gas  flame,  the  refrangibilities  become 
lower  and  lower ;  the  radiant  heat  possessing  a  calorific  tint,  which  successively  de- 
scends through  the  colours  of  the  spectrum,  the  violet,  indigo,  blue,  green,  yellow,  &c. 
From  the  sun,  the  temperature  of  which,  therefore,  nmst  be  excessively  high,  radiant 
heat  is  emitted  which  occupies  a  region  in  the  spectrum  corresponding  to  the  red  rays, 
and  even  below  that  is  found  beyond  the  region  where  red  light  has  ceased  to  be  visi- 
ble. In  the  sunbeam,  therefore,  rays  of  heat  of  every  refrangibility  and  every  colour 
are  found ;  they  occupy  a  space  commencing  with  a  region  beyond  the  extreme  violet, 
and,  descending  through  the  whole  length  of  the  spectrum,  are  found  beneath  its  lowest 
extremity. 

269.  This  is  a  phenomenon  analogous  to  what  we  witness  under  similar  circum- 
stances in  the  case  of  light.  When  a  lamp,  the  wick  of  which  is  placed  very  low,  is 
first  lighted,  it  burns  with  a  violet-coloured  flame,  giving  forth  little  heat,  and  possess- 
ing small  illuminating  power.  13y-and-by,  as  the  combustion  goes  on,  the  colour  passes 
through  various  shades  of  indigo,  and  presently  becomes  of  a  purer  blue.  If  the  wick 
is  now  elevated,  and  air  more  abundantly  supplied,  the  light  increases  in  brilliancy ; 
and,  if  seen  through  a  prism,  all  the  colours  begin  to  be  perceptible  from  the  violet  to 
the  yellow  and  orange.  Lastly,  if  fed  with  oxygen  gas,  or  consumed  in  one  of  the  im- 
proved burners,  the  light  assumes  a  beautiful  whiteness,  and,  if  dispersed  by  a  prism, 
exhibits  all  the  colours  of  the  spectrum.  It  is  to  the  presence  of  these  that  its  white- 
ness is  due,  for  white  light  contains  all  the  coloured  rays. 

270.  In  these  beautiful  and  perfect  analogies,  which  may  thus  be  traced  between  the 
phenomena  of  light  and  heat,  there  are  some  points  which  require  to  be  considered  in 
the  case  we  have  before  us.  As  common  observation  assures  us,  rays  of  light  of  dif- 
ferent refrangibilities  excite  in  our  eyes  specific  sensations ;  the  most  refrangible  ray 
affects  us  with  that  sensation  which  produces  in  the  mind  the  idea  of  a  violet  colour; 
the  middle  refrangible  ray,  a  yellow ;  the  lesser  refrangible  ray,  a  red.  And  now  these 
impressions,  thus  passing  along  the  optic  nerve  to  the  brain,  originate  in  specific  changes 
which  are  happening  to  the  constitution  of  the  retina,  for  this  delicate  expansion  must, 
in  the  nature  of  things,  be  acted  upon  under  the  influence  of  the  light,  in  order  to  give 
rise  to  a  mental  sensation.  The  different  rays  of  light,  each  one  for  itself,  operates  in 
its  own  way  and  produces  its  proper  result. 

271.  Considerations  like  these  would,  therefore,  lead  us  to  suppose  that  rays  of  heat 
of  different  invisible  colours  ought  to  have  the  property  of  producing  specific  changes. 

K 


74 


IT  IS  NOT  RADIANT  HEAT  NOR  THE  'I'lTHOXIC  RAY. 


To  them  all  is  apparently  given  a  power  of  producing  expansion  in  bodies,  and  to  each 
one,  probably,  its  own  specific  chemical  powers.  The  experiments  of  Sir  J.  Hers- 
CHEL  seem  already  to  give  proof  of  this  fact. 

That  the  decomposition  of  carbonic  acid  by  leaves  is  not  due  to  yellow  heat,  may 
be  proved  by  causing  the  active  light  to  pass  through  a  solution  of  bichromate  of  pot- 
ash, which  is  of  an  orange-yellow  colour.  This  ray,  thus  treated,  appears  to  carry  on 
the  decomposition  with  nearly  the  same  activity  as  the  direct  solar  beam  (Ap.,  788). 
In  an  experiment  which  I  made,  using  it  in  a  stratum  of  certain  thickness,  it  seemed  to 
transmit  the  yellow  and  orange  light  with  very  little  loss ;  but  acting  more  energetically 
on  the  calorific  ray,  it  transmitted  of  it  only  -26.  Had  this  heat  been  the  cause  of  the 
decomposition,  the  rapidity  with  which  the  action  took  place  should  have  been  pro- 
portionally reduced. 

272.  From  such  results,  it  is  to  be  inferred  that  radiant  heat  generally,  and  the  yel- 
low rays  of  heat  especially,  do  not  produce  the  decomposition  of  carbonic  acid  gas  in 
the  structure  of  vegetable  leaves. 

273.  This  narrows  the  question  down  to  the  inquiry,  whether  it  be  the  yellow  ray 
of  light  or  the  yellow  tithonic  ray ;  for,  as  has  been  observed,  the  phosphorescent  rays 
may  be  left  out  of  the  discussion. 

274.  Experiments  conducted  on  the  same  principle,  and  nearly  in  the  same  way, 
with  that  already  cited,  serve  to  determine  this  point.  If  a  beam  which  has  passed 
through  a  solution  of  bichromate  of  potash  retains  its  power,  as  we  know  by  experi- 
ment, it  remains  for  us  to  inquire  whether  that  solution  acts  as  feebly  on  the  yellow 
tithonic  ray  as  it  does  on  the  yellow  ray  of  light.  We  have  already  shown  (251) 
that  this  is  not  the  case,  for  the  tithonic  ray  undergoes  an  abundant  absorption,  its 
force  being  greatly  diminished,  so  that  bromide  of  silver,  which  is  easily  changed  by 
the  yellow  tithonic  ray,  undergoes,  in  this  disturbed  and  absorbed  beam,  a  slow  decom- 
position. In  the  same  way,  therefore,  that  we  determine  that  it  could  not  be  radiant 
heat,  we  also  determine  against  the  tithonic  rays.  The  decomposition  of  carbonic 
acid,  the  production  of  chlorophyl,  and  the  greening  of  plants  go  on  with  great  rapid- 
ity under  that  yellow  solution,  because,  although  it  has  absorbed,  and  therefore  removed 
these  imponderable  agents,  it  allows  the  active  light  to  pass  with  little  or  no  diminution. 

275.  The  observation  made  in  237  is  now  understood.  Analysis  by  the  prism 
serves  only  to  point  out  in  what  particular  region  of  the  spectrum  given  phenomena 
are  produced  ;  it  therefore  narrows  our  discussions  down  within  certain  limits.  By 
introducing,  in  addition,  the  action  of  absorbent  media,  we  are  enabled  to  point  out, 
with  a  certain  amount  of  precision,  the  exact  agent  which  is  involved.  This  use  of 
absorbent  media  conjointly  with  prismatic  analysis,  which  was  introduced  into  these 
inquiries  in  1837  (Ap.,  Ch.  X.),  may  be  expected  continually  to  yield  interesting  results. 

276.  To  the  light,  and  more  especially  to  the  yellow  light  of  the  sun,  we  are  to 
impute  the  production  of  this,  the  most  interesting  phenomenon  of  organic  chemistry. 
Rays  which  come  from  artificial  sources,  such  as  lamps  and  gas  flames,  can  also  bring 
it  about  to  a  degree  corresponding  with  their  intensity. 

277.  Whether  that  peculiarity  of  light  by  which  it  gives  to  us  the  sensation  of  spe- 


DECOMPOSITION  OF  THE  RETINA  AND  OTHER  CARBON  COMPOUNDS. 


75 


cific  colours  is  involved,  remains  as  yet  undetermined ;  we  cannot  say  whether  the 
quality  by  which  we  are  led  to  impute  to  a  given  beam  a  yellow  colour  is  the  same 
quality  which  is  involved  in  this  decomposition.  Whether,  in  short,  yellow  light,  be- 
cause it  is  yellow  light,  produces  this  change.  Would  any  other  coloured  ray,  such  as 
a  blue,  if  its  intensity  were  sufficiently  elevated,  produce  the  same  result  t  For  we  can 
imagine  a  blue  light  so  to  be  re-enforced  as  to  possess  the  same  intrinsic  brilliancy  or 
illuminating  power  as  a  yellow.  Under  such  a  change,  would  its  decomposing  action 
also  be  exalted  ? 

278.  Prismatic  experiments  serve  to  show  (Ap.,  782)  that  the  rapidity  of  decompo- 
sition follows  very  closely  the  order  of  illuminating  power.  And  this  result  affords  an 
argument,  imperfect  and  feeble  it  is  true,  that  an  affirmative  answer  will  be  hereafter 
given  to  that  question. 

279.  But  there  are  other  reflections  which  naturally  arise,  and  tend  to  an  opposite 
result.  There  seems  to  be  a  general  relation,  though  the  details  of  it  have  not  yet  been 
traced,  between  rays  of  a  particular  refrangibility  and  ponderable  substances  of  a  par- 
ticular kind.  Thus,  in  the  case  of  most  of  the  salts  of  silver,  the  point  of  maximum 
action  falls  in  the  violet  ray.  In  the  same  way  the  question  naturally  arises.  Does  the 
point  for  the  maximum  action  on  carbon  compounds  fall  in  the  yellow  space,  and  the 
yellow,  for  that  reason,  become  the  active  ray  in  decomposing  carbonic  acid,  and  giv- 
ing a  green  colour  to  leaves  1  Is  it  for  this  cause,  also,  that,  received  into  the  eye,  the 
yellow  ray  impresses  us  with  the  greatest  illuminating  power  ?  It  would  be  a  beauti- 
ful result  of  these  researches  to  co-ordinate  phenomena  apparently  so  widely  apart  as 
the  formation  of  chlorophyl  in  a  leaf  and  the  regulated  destruction  of  the  retina  in  the 
chamber  of  the  human  eye  in  producing  the  phenomena  of  vision.  In  nature  there 
are  many  results  which  are  apparently  equally  distinct,  and  which  the  progress  of 
knowledge  has  shown  are  intimately  allied.  That  to  our  organs  of  vision  yellow  light 
is  the  most  brilliant,  arises  from  the  incidental  circumstance  that  it  is  a  carbonaceous 
compound  of  which  the  changing  nervous  expansion  is  constructed.  Had  it  been  pos- 
sible for  Nature  to  have  formed  a  retina  in  which  a  sah  of  silver  formed  the  basis,  the 
maximum  of  brilliancy  of  light  would  have  shifted,  and  the  blues  would  have  been 
among  the  brightest  rays.  Is  it  in  the  optical  peculiarities  of  the  carbon  atom  that  all 
our  ideas  of  harmony  among  colours  and  beauty  of  external  objects  have  arisen  1 

280.  Experimental  science  will  probably  before  long  trace  a  close  connexion  between 
the  physical  properties  of  atoms  and  the  physical  properties  of  hght.  It  will  show  that 
molecules  of  a  given  weight  can  be  moved  most  easily  by  ethereal  waves  of  a  given 
length,  as  a  stretched  string  is  thrown  into  vibration  by  atmospheric  undulations  of 
proper  dimensions;  that  the  transverse  vibrations  of  the  ethereal  particles  can  agitate 
in  a  corresponding  way  ponderable  atoms  of  a  proper  magnitude  and  constitution.  We 
shall  then  have  no  difficulty  in  understanding  how  it  was  that  among  metallic  sub- 
stances, those  first  detected  to  be  changed  by  hght,  such  as  silver,  gold,  mercury,  lead, 
have  all  high  atomic  weights ;  and  such  as  sodium  and  potassium,  the  atomic  weights 
of  which  are  low,  appeared  to  be  less  changeable. 


76 


THEORY  OF  ABSORPTION. 


CHAPTER  IX. 

THEORY  OF  THE    ABSORPTION  OF  THE  TITHONIC  RAYS  AND  LIGHT. 

Contents  :  Estimate  of  the  Extent  andPoimr  of  the  Solar  Radiations.— Influence  still 
exists  in  the  Moonbeams. — Absorptive  Action  of  Chlorine  and  Hydrogen. — Detitho- 
nization  of  the  Ray  and  Tithonization  of  the  Gaseous  Mixture. — Curve  and  Law. — 
Deductions  as  to  Latent  Light  and  Definite  Action. — Functions  discharged  by  the 
Chlorine  and  Hydrogen  respectively. 

281.  In  pursuing  our  discussion  of  the  phenomenon  which  we  have  under  consider- 
ation— tlie  digestion  of  plants — we  have  successively  traced  the  source  of  action  to  the 
jellow  region  of  the  spectrum,  and  to  the  ray  of  light. 

282.  In  what  manner,  then,  does  this  light  act  ?  How  does  it  come  to  pass  that  it 
can  exert  so  great  a  force  as  to  effect  the  reduction  of  carbonic  acid  in  the  cold  ?  We 
have  briefly  seen  what  are  the  results  it  impresses  on  the  forming  vegetation,  that  the 
leaf  turns  green  and  oxygen  is  given  off.  What  are  the  corresponding  and  contem- 
poraneous changes  which  happen  to  the  light?  Action  and  reaction  are  always  equal, 
and  if  a  given  beam  can  produce  a  result  which  demands  the  most  energetic  chemical 
force,  it  is  reasonable  to  suppose  that  in  doing  so  it  undergoes  itself  a  change. 

283.  These  considerations  show  us  that  the  question  in  what  manner  yellow  light 
acts  in  controlling  the  function  of  digestion  in  plants,  is  not  only  exceedingly  interest- 
ing in  a  physiological  point  of  view,  but  also  that  it  involves  the  whole  theory  of  the 
action  of  radiant  matter,  whether  it  be  of  light,  heat,  or  tithonicity,  in  producing  chemi- 
cal change. 

284.  There  are  few  authors  who  have  written  on  the  action  of  light  or  the  tithonic 
rays  in  producing  chemical  changes,  who  have  not  directly  ascribed  all  those  changes 
to  absorption  of  the  imponderable  principle.  The  connexion  between  absorption  and 
the  production  of  these  phenomena  is  clearly  apparent.  Still,  however,  in  looking  over 
what  has  been  written,  we  find  little  precision  in  those  views ;  instead  of  a  distinct  con- 
ception of  a  plain  fact,  we  find  only  loose  and  imperfect  ideas. 

285.  In  animals,  voluntary  and  involuntary  motions  are  under  the  government  of  the 
nervous  system.  Each  movement  which  is  executed  is  attended  with  a  corresponding 
consumption  of  organized  matter,  either  in  the  muscular  or  nervous  tissues,  or  both. 
The  motions  which  my  fingers  are  executing  in  writing  these  lines  do  not  spring  forth 
from  nothing,  but  are  the  offspring  of  the  destruction,  in  a  regulated  manner,  of  organ- 
ized matter  originally  derived  from  food.  How  perfect,  then,  is  the  animal  machine, 
which,  fed  from  day  to  day  by  a  small  portion  of  carbonaceous  matter,  executes  motions 
with  an  inconceivably  small  expenditure  of  material !  How  great,  also,  are  the  results 
which  may  arise  from  the  return  of  those  organized  atoms  to  their  pristine  inorganic 


AMOUNT  OF  FORCE  IN  THE  SOLAR  RADIATIONS.  77 

State.  Thrown  out  from  the  mechanism,  after  their  office  is  over,  they  leave  behind 
them  marks  of  the  changes  through  which  they  have  passed,  and  of  the  facts  to  which 
they  have  given  birth,  and  thus  stand  at  last  in  connexion  with  events  to  which  at  first 
they  were  not  apparently  allied. 

286.  From  the  combustion  of  small  quantities  of  carbon,  we  see,  in  improved  steam- 
engines,  how  great  an  amount  of  force  can  be  originated,  and  by  the  oxydation  of  a  few 
grains  of  zinc  in  voltaic  batteries,  what  surprising  chemical  results  arise.  From  those 
more  ordinary  cases  of  changes  accomplished  by  the  action  of  light,  which  appear  to 
be  feeble  and  slowly  produced,  we  should  form  the  most  erroneous  opinions  of  the  force 
of  the  sun  rays.  General  considerations  might  lead  us  to  know  that  the  principle 
which  has  in  charge  the  keeping  up  of  the  constitution  of  the  atmosphere,  and  regula- 
ting the  vital  functions  of  plants,  is  of  great  intensity.  Thus,  I  have  found  that  the 
rays  which  are  emitted  from  a  common  wax  candle  are  superior  in  chemical  force  to 
the  current  which  is  evolved  by  a  cell  of  Grove's  battery,  the  most  powerful  of  voltaic 
combinations  known,  for  they  could  effect  the  recomposition  of  muriatic  acid  nmch 
faster  than  the  battery  could  decompose  it,  and  yet  that  battery  was  found  competent 
to  maintain  a  platina  wire  white  hot,  and,  if  the  views  of  Dr.  Faraday  are  correct,  was 
evolving  more  electricity  than  is  developed  by  any  thunder  storm.  If  this  is  the  case 
with  a  candle,  what,  then,  shall  we  say  of  brilUant  rays  of  the  sun,  which  impinge  on  the 
earth  on  all  sides  1 

287.  That  force,  therefore,  the  mode  of  action  of  which  we  are  now  to  discuss,  is 
far  from  an  insignificant  power  in  Nature.  In  generality  and  intensity  it  rivals  any  that 
is  known;  in  interest  it  is  superior  to  them  all,  for  it  stands  in  connexion  with  organiza- 
tion and  living  things. 

288.  Even  after  having  undergone  that  enormous  reduction  of  intensity  which  must 
take  place  in  reflexion  from  the  surface  of  the  moon,  the  solar  rays  still  act  with  energy ; 
for  the  moonshine  produces  all  kinds  of  decompositions,  acting  like  the  sunbeam, 
though  in  a  feeble  way,  being  probablv  reduced  to  ^oao-uo  P^^^  of  its  original  power  (Ap., 
545). 

289.  This  force,  thus  affecting  in  a  radiant  form  vegetable  organizations,  produces 
he  results  we  are  studying.    The  idea  commonly  entertained  of  its  feebleness  is  utterly 
inaccurate  and  wrong. 

290.  In  proceeding  now  with  the  more  immediate  object  of  this  chapter,  I  shall 
follow  the  course  of  thought  which  has  presented  itself  in  my  experiments.  At  the 
risk  of  dwelling  somewhat  in  tedious  detail,  I  shall  also  describe  the  different  experi- 
ments from  which  the  final  arguments  are  drawn  ;  and  as  it  will  be  found  that  the 
rays  of  light  and  the  tithonic  rays  are  eventually  under  the  government  of  the  same 
laws,  similar  expressions  including  the  phenomena  of  both,  I  shall  commence  with  giving 
the  theory  of  the  absorption  of  the  tithonic  rays  first,  and  then  show  how  the  same 
theory  includes  the  phenomena  of  light.  This  leads  naturally  to  a  division  of  the  sub- 
ject under  two  heads. 

I.  Theory  of  absorption  of  the  tithonic  rays. 

II.  Theory  of  ideal  coloration. 


78 


ABSORPTIVE  ACTION  OF  CHLORINE  AND  HYDROGEN. 


To  the  first  of  these  the  present  chapter  is  devoted ;  the  following  chapter  to  the 
second. 

291.  When  a  beam  of  light  has  fallen  on  any  changeable  surface,  such  as  a  Daguer- 
reotype plate,  and  is  reflected  by  it,  that  beam  will  be  found  to  have  impressed  a  change 
on  the  sensitive  surface,  greater  or  less  in  amount,  according  to  the  period  of  its  aciion. 
In  effecting  this  it  also  suffers  a  change  itself,  and  if  received  on  a  second  similar  sensi- 
tive surface,  is  found  to  have  lost  the  quality  of  giving  rise  to  the  decomposition  again. 
Two  changes  have,  therefore,  occurred,  a  change  in  the  ponderable  body,  and  a  change 
in  the  incident  beam.  The  particular  experiments  in  proof  of  this  fact  are  given  in 
Ap.,  595. 

292.  Again,  let  us  take  a  second  instance.  As  is  shown  in  Ap.,  Ch.  XVI.,'  a  mix- 
ture of  chlorine  and  hydrogen  in  equal  volumes  undergoes  combination  by  the  influ- 
ence of  the  rays  of  a  lamp,  and  a  rapid  action,  amounting  to  an  explosion,  by  the 
brighter  beams  of  the  sun.  As  these  gases  can  be  obtained  in  a  state  of  uniform  purity, 
and  tlieir  combination  is  attended  with  mechanical  results,  this  forms  a  favourable  case 
for  a  minute  investigation.  With  one  such  clear  case  to  guide  us  in  our  researches,  we 
may  fall  back  on  it  for  illustrations,  as  new  phenomena  arise. 

293.  A  mixture  of  chlorine  and  hydrogen,  such  as  has  been  referred  to,  was  placed 
in  a  vessel  made  of  plate  glass,  having  flat  and  parallel  sides;  it  was  7  inches  high,  2 
broad,  and  2'6  deep.  It  was  so  arranged  on  a  small  porcelain  trough,  that  it  could  be 
used  as  a  gas  jar.  The  rays  of  an  argand  lamp,  properly  situated,  were  made  to  pass 
through  it ;  they  therefore  went  through  a  depth  of  the  compound  gases  of  2*6  inches.  In 
Jig.  123,  A  is  the  lamp,  so  adjusted  as  to  burn  steadily,  B  the  vessel  containing  the 
chlorine  and  hydrogen,  C  the  porcelain  trough,  in  which  was  placed  a  saturated  solu- 
tion of  common  salt,  which  acts  on  the  chlorine  slowly,  and  therefore  allows  us  to 
make  any  necessary  experiments  without  much  change  happening  in  the  gases  under 
trial.  At  D  was  placed  a  tithonometer  (Ap.,  Ch.  XVI.),  to  receive  the  rays  from  the 
lamp,  after  they  had  emerged  from  the  chlorine  and  hydrogen. 

294.  Two  separate  phenomena  were  now  apparent :  first,  the  mixture  of  chlorine 
and  hydrogen  began  to  unite  under  the  influence  of  the  rays  of  the  lamp ;  second, 
the  rays  which  had  passed  through  the  mixture  had  lost  very  much  of  their  chemical 
power.  It  was  not  totally  extinct,  but  the  tithonometer  showed  that  it  had  undergone 
a  very  great  diminution. 

295.  We  see,  therefore,  that  on  its  passage  through  a  mixture  of  chlorine  and  hy- 
drogen, the  beam  has  become  detithonized.  Simultaneously,  and  in  producing  this  re- 
sult, we  see  that  the  sensitive  mixture  has  become  tithonized.  The  connexion  and 
sequence  of  the  phenomena  are  apparent.  The  beam  has  undergone  a  change  itself 
in  producing  a  given  change  in  the  ponderable  matter.  But  this  is  the  same  conclu- 
sion that  was  furnished  us  by  the  rougher  experiment  with  iodide  of  silver  above  quo- 
ted (Ap.,  595). 

296.  At  this  stage  of  our  inquiries,  therefore,  we  have  already  fallen  upon  one  of  the 
leading  features  of  the  doctrine  of  absorption  ;  for  we  perceive  that,  whenever  tithonic 
rays  produce  a  change  on  a  sensitive  mixture,  they  must  necessarily  undergo  a  change 
themselves,  and  become  partially  or  perfectly  detithonized. 


PARTICULAR  PHENOxMENA  EXHIBITED  BY  CHLORINE  AND  HYDROGEN.  79 

297.  From  this  point  our  inquiries  naturally  brancii  in  two  directions.  First,  The 
consideration  of  what  happens  to  the  substance  which  is  thus  in  the  act  of  being  chan- 
ged or  tithonized.  Second,  What  happens  to  the  ray  in  undergoing  its  converse  change, 
or  being  detithonized.    These  I  shall  discuss  in  succession. 

298.  Phenomena  of  the  Tithonization  of  CIdorine  and  Hydrogen. — The  tithonome- 
ter  enables  us  to  ascertain  the  leading  phenomena  in  a  very  satisfactory  manner.  Its 
sensitive  material  being  (Ap.,  838)  the  very  mixture  the  properties  of  which  we  are 
considering,  to  determine  the  changes,  and  the  rapidity  of  the  changes  which  take  place 
in  that  mixture,  we  are  only  required  to  place  a  tithononietcr  in  the  rays  of  a  lamp, 
remove  all  external  sources  of  disturbance,  such  as  the  action  of  radiant  heat,  &c.  and 
note  the  results. 

299.  To  carry  otit  these  views,  I  have  employed  the  arrangement  represented  in^jg-. 
124.  A  is  an  argand  lamp,  which,  during  the  period  of  observation,  burns  with  uni- 
formity. In  front  of  this,  and  at  a  distance  of  about  two  inches,  an  arrangement  of 
double  convex  lenses,  B,  is  placed.  Beyond,  at  a  distance  of  7  inches,  is  a  second 
convex  lens,  3-5  inches  focus.  Between  B  and  C,  a  metallic  screen,  E,  is  arranged, 
so  that  it  can  be  easily  removed  or  replaced,  according  as  it  is  desired,  to  allow  the 
rays  of  the  lamp  to  fall  upon  the  tithononieter,  or  to  cut  them  off. 

300.  The  mode  of  action  of  the  lenses  of  this  arrangement  is,  to  give  a  uniform 
disc  of  light,  M,  on  the  sentient  tube  of  the  tithononieter.  When  a  piece  of  white  pa- 
per is  placed  so  as  to  receive  this,  in  front  of  the  instrument,  there  is  a  circular  disc, 
which  is  equally  luminous  all  over.  If  this  condition  be  not  exactly  fulfilled,  the  lamp 
or  the  lenses  are  to  be  moved  and  adjusted  until  the  illumination  is  sensibly  the  same. 
We  have  then  the  sentient  tube  of  the  tithononieter  plunged  in  an  area  of  light  which 
remains  uniform  in  intensity  during  the  period  of  our  researches. 

301.  In  this  invariable  disc  of  light  we  have  to  expose  the  mixture  of  chlorine  and 
hydrogen,  and  mark  on  the  scale  of  the  tithononieter  the  progress  of  its  union.  This 
we  do  by  noticing  how  many  seconds  elapse  before  the  contraction  arising  from  the 
production  of  muriatic  acid  begins,  and  then  how  many  seconds  elapse  as  the  liquid  in 
the  index  tube  passes  over  each  division. 

302.  In  a  particular  experiment  of  this  kind,  the  following  numerical  determinations 
were  obtained  : 

303.  On  removing  the  screen  E,  and  allowing  the  rays  to  fall  on  the  sensitive  mix- 
ture, first  of  all  an  expansion,  amounting  to  half  a  degree  upon  the  scale,  was  observed. 
In  sixty  seconds  this  expansion  ceased. 

304.  The  mixture  now  remained  stationary,  no  apparent  change  going  on  in  it. 
At  length,  after  the  close  of  270  seconds,  it  was  beginning  to  contract,  and  muriatic 
acid  to  form. 

305.  At  the  end  of  45  seconds  more,  a  contraction  of  half  a  degree  had  been  accom- 
plished ;  the  dimensions  of  the  mixture  were  therefore  now  the  same  as  when  the  ex- 
periment first  began  ;  this  half  degree  of  contraction  compensating  for  the  half  de- 
gree of  expansion. 

306.  The  number  of  seconds  which  elapsed  as  the  liqnid  descended  over  the  scale, 


/ 


80  PARTICULAR  PHENOMENA  EXHIBITED  BY  CHLORINE  AND  HYDROGEN. 

through  contraction  of  the  sentient  gases,  was  now  determined.  These  numbers  are 
contained  in  the  following  table.  In  one  column  is  given  the  number  of  each  divis- 
ion ;  in  the  adjacent  one,  the  period  of  contraction  through  it. 


Spaces. 

Time  in  Seconds. 

Spaces. 

Time  in  Seconds. 

Kxpanded  ^  degree  in     .    .  . 

60 

16 

18 

Movement  commenced  . 

270 

17 

18 

Reached  J  degree  of  expansion 

45 

18 

18 

1 

55 

19 

18 

2 

40 

20 

18 

3 

28 

21 

17 

4 

27 

22 

17 

5 

25 

23 

18 

6 

23 

24 

17 

7 

22 

25 

18 

8 

20 

26 

18 

9 

20 

27 

17 

10 

21 

28 

16 

11 

20 

29 

16 

12 

20 

30 

16 

13 

20 

31 

16 

14 

19 

32 

16 

15 

19 

Let  us  now  take  these  observations  and  project  them,  as  is  done  in  Jig.  125,  the 
amount  of  contraction  representing  the  quantities  of  gases  that  have  united  being  laid 
off  on  the  axis  of  an  abscissas,  the  times  on  the  ordinates.  But  these  times  represent  the 
number  of  rays  which  have  fallen  on  the  sentient  mixture  ;  consequently,  the  ordinates 
of  that  curve  represent  the  quantities  of  tithonic  rays,  and  the  abscissas  the  corre- 
sponding chemical  effects. 

307.  Inspection  of  the  curve  shows  its  peculiarities  at  once.  We  see  that  after  the 
first  preliminary  expansion  has  taken  place,  expressed  by  that  portion  between  a  and  h, 
for  a  certain  space  of  time,  although  rays  are  constantly  falling  on  the  mixture  and  be- 
ing absorbed,  no  visible  effect  is  produced,  there  being  neither  expansion  nor  contraction, 
as  is  shown  hy  h  c.  A  certain  space  of  time,  amounting  in  this  instance  to  270  seconds, 
being  now  accomplished,  contraction,  from  union  of  the  gases,  begins.  In  that  portion 
of  the  curve  c  d  which  represents  the  progress  of  the  phenomenon,  the  curvature  is  per- 
petually diminishing,  and  at  d  approaches  sensibly  to  a  straight  line.  From  d  to  e,  which 
includes  the  remainder  of  the  observations,  the  line  preserves  its  rectilinear  character. 

308.  The  study  of  the  properties  of  this  curve,  or  of  the  tabular  numbers,  serves  to 
prove  that  when  chlorine  and  hydrogen  unite  under  the  influence  of  the  tithonic  rays, 
there  are  four  distinct  periods  of  action. 

1st.  For  a  brief  space  the  mixture  expands. 

2d.  For  a  much  longer  period  it  then  remains  wholly  stationary,  neither  expanding 
nor  contracting,  although  the  rays  are  constantly  falling  on  it  and  it  is  absorbing  them. 

3.  Contraction  arising  from  the  production  of  muriatic  acid  begins,  commencing  at 
first  slowly,  then  more  and  more  rapidly. 

4th.  And  after  that  contraction  has  fairly  set  in,  it  goes  on  with  uniformity,  equal 
quantities  of  muriatic  acid  being  produced  in  equal  times  by  the  action  of  equal  quan- 
tities of  the  rays. 

309.  If,  therefore,  it  is  permitted  us  to  generalize  from  this  case  of  the  action  of  rays 
on  one  of  the  most  sensitive  substances  known,  a  mixture  of  chlorine  and  hydrogen,  we 


LATENT  LIGHT.  g] 

should  assert  that  when  a  ray  falls  on  a  sensitive  compound,  its  first  effect  is  to  pro- 
duce an  expansion.  During  a  certain  period,  differing  in  different  cases,  no  charge 
whatever  lakes  place,  the  ray  being  constantly  absorbed,  and  not  appearing  to  produce 
any  visible  effect.  After  a  time,  chemical  action  commences,  at  first  more  slowly,  then 
with  a  determined  and  constant  rate  of  rapidity,  equal  quantities  of  the  rays  now  pro- 
ducing equal  chemical  effects. 

310.  I  may  here  anticipate  what  will  presently  be  proved,  that  this  generalization 
can  be  sustained,  and  that,  therefore,  the  conclusions  which  we  have  arrived  at  for  this 
particular  case,  hold  also  for  all  others,  not  only  in  the  cases  of  chemical  action  pro- 
duced by  the  tithonic  rays,  but  also  of  chemical  action  brought  about  by  the  rays  of 
light. 

311.  Let  us  return  again  to  the  study  of  the  curve,  or  of  the  numbers  contained  in 
the  table.  It  is  obvious  that  there  are  two  portions  of  these  which  demand  peculiar 
attention;  they  are  embraced  in  the  second  and  fourth  epochs  referred  to  in  (308).  As 
to  the  first  and  third,  these  are  for  the  present  of  less  interest. 

312.  What,  then,  is  the  interpretation  we  are  to  pitt  upon  that  part  of  the  curve 
which  is  between  h  and  c  1  or,  in  other  words,  what  is  the  interpretation  we  are  to 
give  of  the  fact,  that  when  a  sensitive  compound  is  exposed  to  a  given  ray,  it  does 
not  change  all  at  once,  but  a  certain  period  must  elapse  during  which  absorption  is  go- 
ing forward,  without  any  corresponding  apparent  effect  ensuing,  and  that  once  accom- 
plished, chemical  change  begins? 

313.  Is  not  this  the  same  phenomenon  which  has  been  for  a  long  time  known  in  the 
case  of  radiant  heat  \  When  a  ray  of  heat  falls  on  a  mass  of  ice  at  32°  Fah.,  in  which 
a  thermometer  is  imbedded,  for  a  certain  space  of  time  no  apparent  rise  of  temperature 
takes  place,  but  the  radiation  continuing  long  enough,  a  physical  change  is  accomplish- 
ed ;  the  ice  puts  on  a  fluid  form,  and  now  the  thermometer  commences  to  ascend, 
equal  quantities  of  heat  producing,  for  a  certain  period,  proportionally  equal  effects. 
Woidd  not  the  table  given  in  (306),  or  the  curve  projected  in  jig.  125,  answer  as  well 
to  express  the  phenomenon  of  the  action  of  caloric  upon  ice,  as  of  the  tithonic  rays  on 
a  mixture  of  chlorine  and  hydrogen  ? 

314.  It  was  from  the  study  of  that  phenomenon  in  the  case  of  ice  that  the  doctrine 
of  latent  heat  arose  ;  and  do  not  these  things  teach  us  that  just  as  a  calorific  ray  becomes 
latent  under  certain  circumstances,  so  also  does  a  tithonic  ray,  and,  consequently,  a  ray 
of  light  \  I  regard  the  phenomenon  of  that  pause  which  is  seen  before  chlorine  and 
hydrogen  unite,  and  during  which  absorption  is  taking  effect,  as  setting  forth  in  a  strong, 
and  clear,  and  prominent  manner,  that  as  radiant  heat  may  become  latent,  so  also  may 
tithonic  rays,  and  also  rays  of  light. 

315.  Let  us,  in  the  next  place,  direct  our  attention  to  the  second  branch  of  the  curve, 
or  to  the  fourth  epoch,  the  changes  of  which  are  included  between  d  and  e.  Rigor- 
ously speaking,  this  is  not  a  straight  line.  It  only  makes  a  sensible  approach  to  one. 
There  are  several  causes  which  obviously  interfere.  When  a  given  quantity  of  gase- 
ous mixture  is  exposed  to  a  radiant  source,  and  the  experiment  we  have  been  relating 
performed,  it  is  obvious  that,  as  it  proceeds,  the  volume  of  gas  so  exposed  steadily  di- 

L 


82 


PRELIMINARY  ABSORPTION. 


minishes,  and  the  effect  of  this  must  be  apparently  to  dimmish  the  force  of  the  ray. 
Moreover,  during  the  progress  of  the  trial,  muriatic  acid  gas  accumulates,  for  its  ab- 
sorption does  not  instantaneously  take  place  ;  the  operation  of  this  must  be  the  reverse 
of  the  former,  or  apparently  to  increase  the  effect.  To  a  certain  extent,  but  not  per- 
fectly, these  sources  of  error  may  be  avoided,  as  I  have  attempted,  and  practically  we 
may  discuss  the  branch  of  the  curve,  d  e,  as  though  it  were  a  straight  hne. 

316.  This  being  understood,  the  result  which  is  before  us  leads  us  obviously  to  this 
important  law,  that  for  a  given  compound,  equal  quantities  of  tithonic  rays,  after  the 
preliminary  latent  absorption  is  over,  give  rise  to  equal  chemical  effects. 

317.  In  thus  setting  forth,  in  as  prominent  a  manner  as  I  am  able,  these  two  doctrines, 
1st.  Of  the  latent  condition  of  the  rays  that  are  at  first  absorbed  ;  and,  2d.  Of  the  def- 
inite chemical  action  of  those  that  are  subsequently,  I  am  again  urging  the  same  doc- 
trine which  three  years  ago  (Ap.,  595)  I  attempted  to  establish  for  iodide  of  silver. 

318.  In  passing,  it  may  be  observed,  that  one  of  the  greatest  practical  difficulties  in 
the  art  of  photography,  more  especially  in  taking  portraits  from  life,  is  connected 
with  this  matter  of  preliminary  latent  absorption.  Artists  know  well,  that  to  obtain  a 
perfect  result  is  the  exception,  and  an  indifferent  one  is  the  rule.  It  is,  indeed,  rare 
that  the  relation  of  light  and  shadow  is  perfectly  observed ;  the  high  lights  most  com- 
monly come  out  unduly,  the  feebler  lights  are  more  slowly  evolved,  and  very  often  never 
come  out  at  all.  This  arises  from  the  circumstance  that  the  sensitive  surface  does  not 
begin  to  change  uniformly  from  the  first  instant  of  exposure,  but  the  prehminary  latent 
stage  of  absorption  has  to  be  gone  through  ;  with  high  lights  and  a  brilliant  illumination, 
that  period  is  passed  over  in  an  instant,  and  the  second  entered  upon;  but  the  feebler 
fights  have  to  expend  themselves  for  a  long  while  in  passing  through  this  stage,  and 
while  the  others  have  carried  on  their  operation  almost  to  completion,  these  have  not 
been  able  to  leave  a  sensible  trace  of  action.  Theoretically,  the  remedy  for  this  diffi- 
culty would  be,  by  a  brief  exposure  to  a  transitory  or  dim  light,  to  pass  the  sensitive 
surface  uniformly  through  its  preliminary  stage. 

319.  These,  therefore,  are  the  prominent  phenomena  which  are  exhibited  by  a  mix- 
ture of  chlorine  and  hydrogen,  a  latent  preliminary  absorption,  and  a  subsequent  def- 
inite chemical  action.  We  have  observed  them  also  in  iodide  of  silver  (317),  and  in 
various  other  compounds. 

320.  liCt  us  direct  our  attention,  in  the  next  place,  to  what  has  happened  to  the  ray. 
We  have  already  seen  (295)  that  when,  .through  a  gaseous  sensitive  mixture,  the  beams 
from  a  lamp  are  suft'ered  to  pass,  and  fall  on  the  tithonometer,  they  are  found  to  have 
lost  much  of  their  chemical  force.    The  beam  has  therefore  become  detithonized. 

(a.)  The  vessel  described  (293)  was  filled  with  atmospheric  air  over  the  trough, 
and  the  chemical  force  of  the  ray  passing  through  it  from  the  lamp  {Jig.  123)  was  de- 
termined. It  was  measured  by  the  period  required  to  cause  the  index  to  descend 
through  one  division,  and  represented  by  12  seconds. 

(Z>.)  The  vessel  was  now  half  filled  with  chlorine,  derived  from  a  mixture  of  muriatic 
acid  and  peroxide  of  manganese,  and  the  chemical  force  of  the  ray,  after  passing  through 
it,  determined  us  before  ;  it  now  was  represented  by  251  seconds. 


FUNCTIONS  DISCHARGED  BY  THE  CHLORINE  AND  HYDROGEN  RESPECTIVELY.  33 

(c.)  To  tlie  chlorine  an  equal  volume  of  hydrogen  was  now  added,  the  vessel  being, 
consequently,  full  of  the  united  mixture.  The  force  of  the  ray  was  again  measured, 
and  found  to  be  represented  by  19  seconds. 

(d.)  Lastly,  the  hrst  (a)  of  these  preceding  measures  was  determined  again,  with  a 
view  of  ascertaining  whether  the  intensity  of  the  lamp  had  declined,  or  the  apparatus 
remained  in  its  first  condition.    It  gave  again  12  seconds. 

321.  Let  us  group  these  four  results  together,  representing  thus  the  intensity  of  the 
beam  by  the  time  it  requires  to  produce  a  given  effect: 

A  beam  through  the  glass  vessel   12  seconds. 

Chlorine.   2.5-5  " 

Chlorine  and  hydrogen   19  " 

Atmospheric  air     .   12  " 

We  therefore  gather  from  this,  that  the  addition  of  hydrogen  to  chlorine,  far  from  in- 
creasing its  absorptive  power,  actually  diminishes  it.  That  in  the  case  before  us, 
where  to  a  given  volume  of  chlorine  an  equal  volume  of  hydrogen  has  been  added,  the 
absorptive  power  is  diminished  to  one  half 

322.  We  farther  see,  that  the  action  of  the  beam  is  expended  primarily  on  the  chlo- 
rine, giving  to  it  a  disposition  to  go  into  union  with  hydrogen,  and  that  the  functions 
discharged  by  the  chlorine  and  hydrogen  respectively  are  wholly  different. 

323.  The  chemical  forces  of  the  ray  are  easily  deduced  from  the  foregoing  meas- 
ures, in  which  the  times  are  given,  for  it  is  obvious  that  they  are  inversely  proportional 
to  those  times. 

324.  The  second  and  third  experiments  (b)  and  (c)  may,  without  sensible  error,  be 
taken  as  representing  the  activity  of  the  ray  in  vacuo,  for,  as  will  be  seen,  upon  prin- 
ciples hereafter  given,  a  ray  which  has  passed  through  atmospheric  air  has  not  under- 
gone any  absorptive  action,  and  therefore  does  not  differ  from  one  which  has  passed 
a  vacuum.  Consequently,  those  measures  give  us  the  effect  of  chlorine,  and  of  chlo- 
rine and  hydrogen,  compared  with  a  vacuum.  The  absorptive  action  of  the  glasses  is 
common  to  all  the  experiments,  and  may  therefore  be  left  out  of  the  final  estimate. 
The  difference  of  the  resulting  numbers  in  and  c,  from  the  probable  numbers  25-5 
and  18"7,  may  be  accounted  for  from  the  disturbing  causes  which  are  encountered,  such 
as  the  constant  solution  of  chlorine  by  the  salt  water,  &c. 

325.  When,  therefore,  a  ray  of  light  falls  upon  this  changeable  compound,  chlorine 
and  hydrogen,  the  primary  action  takes  place  upon  the  chlorine,  which  becomes  titho- 
nized,  or  has  a  disposition  given  to  it  to  go  into  union  with  the  hydrogen ;  the  latter 
gas  appears  to  be  passive,  so  far  as  the  ray  is  concerned.  In  the  mean  time,  the  ray 
itself  becomes  changed,  undergoing  ab.sorption  action,  and  being  detithonized. 


84 


THEORY  OF  IDEAL  COLORATION. 


CHAPTER  X. 

THEORY  OF  IDEAL  COLORATION. 

Contents  :  Forme?-  Observations  on  Colours  in  the  Chemical  Rays.  —  Nomeiichitare 
derived  from  it. — Case  of  the  Chrysotype. — Case  of  Bichromate  of  Potash. — Laws 
deduced. — Control  of  Optical  Forces  over  Chemical  Effects. — Ajyplication  to  Spectrum 
Stains. — HerscheVs  Law  for  Light. — Explanation  of  Variable  Ejf  ects  in  Films  of 
different  Thickness. — Mode  of  Action  of  the  Tithonic  Rays. 

326.  In  the  year  1837,  while  the  study  of  the  chemical  agencies  of  light  was  yet  in 
its  infancy,  from  phenomena  connected  with  the  decomposition  of  carbonic  acid,  the 
synthesis  of  chlorine  and  hydrogen,  and  the  decomposition  of  chloride  of  silver,  I  came 
to  the  conclusion  that  there  were  among  the  chemical  rays  intrinsic  differences,  of  the 
same  order  as  the  differences  in  colour  among  the  rays  of  light,  and  that  for  those  rays 
the  doctrine  of  invisible  coloration  would  have  to  be  admitted  (Ap.,  Ch.  X.). 

327.  More  recently,  M.  Melloni,  to  whom  science  is  indebted  for  originating  and  de- 
veloping this  beautiful  thought,  in  the  case  of  rays  of  heat,  has  independently  come  to 
the  same  conclusion.  So  intimately  is  this  idea  bound  up  with  the  explanation  of  the 
phenomena,  that,  in  the  case  of  radiant  heat,  that  eminent  philosopher  proposes  to  use 
it  as  the  foundation  for  the  whole  science,  and  for  its  nomenclature. — {Taylor s  Sc.  Me- 
tnoirs,  vol.  iii.,  p.  12.) 

328.  In  what  follows,  I  shall  not  attempt  to  introduce  the  doctrine  in  an  analytical 
way,  or  to  trace  the  arguments  and  experiments  which  lead  to  its  conclusions  ;  but,  assu- 
ming it  at  once  as  true,  show  with  what  facility  and  ease  it  may  be  brought  to  explain 
a  number  of  remarkable  phenomena.  Calling  to  mind  the  facts  and  views  which  have 
been  given  in  the  preceding  chapter,  and  more  especially  to  the  general  fact,  that  when- 
ever a  ray  produces  a  chemical  effect  it  undergoes  a  change,  the  sensitive  or  changea- 
ble medium  absorbing  some  one  or  more  of  its  constituents,  it  remains  now,  in  addition 
to  the  considerations  there  given  respecting  the  characters  of  the  rays,  to  introduce  an- 
other element — the  element  of  variable  refrangibility. 

329.  Asserting,  therefore,  that  for  the  chemical  rays  there  exist  peculiarities  analo- 
gous to  the  different  colours  of  light — peculiarities  which,  by  reason  of  the  invisibility 
of  those  rays,  are  known  to  us  only  by  certain  chemical  phenomena — it  remains,  in 
bringing  forward  this  doctrine  with  clearness  and  precision,  to  adopt  some  provisional 
nomenclature,  and,  for  want  of  a  better,  one  founded  on  those  terms  heretofore  intro- 
duced (242)  will  serve  our  present  purposes. 

330.  In  developing  the  doctrine  of  ideal  coloration  applied  to  the  tithonic  rays,  I 
shall  therefore  resort  to  what  appears  to  be  the  simplest  rule,  and  designate  rays  of  dif- 
ferent orders  of  refrangibility  by  the  same  nomenclature  which  has  been  used  for  light : 
thus  we  shall  have 


ANALYSIS  OF  THE  CHRYSOTYPE.  85 

Tithouic  red. 

Tithonic  orange. 

Tithouic  yellow,  &c.,  &c., 
the  expressions  pointing  out  the  region  of  the  spectrum  under  consideration,  but 
drawing  a  strong  and  perfect  distinction  between  the  agent  involved  and  light.    In  the 
same  way  other  derivates  will  arise,  such  as  tithonic  white  or  black. 

331.  When  a  beam  of  light  is  dispersed  by  the  action  of  a  prism,  and  the  resulting 
spectrum  examined  by  physical  tests,  it  is  easy  to  recognise  that  the  quality  of  impart- 
ing a  sensation  of  difference  of  colour  is  not  the  only  token  of  intrinsic  difference  in 
the  character  of  rays  of  different  refrangibilities.  Writers  on  the  mechanical  theory  of 
optics  have  assumed,  that  the  constitutional  distinction  between  the  various  colour 
giving  rays  is  the  different  lengths  of  waves  which  they  represent.  To  a  given  index 
of  refrangibility  there  belongs  a  particular  length  of  wave,  and  a  particular  tint  of  col- 
our. But  there  are  facts  connected  with  the  history  of  light  which  seem  to  prove  that 
beyond  this  there  are  peculiarities  which  are  far  more  profound. 

332.  Several  years  ago,  Sir  D.  Brewster  showed  that,  by  resorting  to  absorbent 
media,  rays  of  any  colour  could  be  insulated  in  every  part  of  the  spectrum ;  that  red 
light  existed  in  the  violet  spaces,  and  blue  light  in  the  red.  These  results,  being  sub- 
stantiated, would  appear  to  afford  a  very  formidable  argument  against  the  dependance 
of  colour  on  wave  length. 

333.  I  do  not  propose  to  enter  here  on  any  speculative  considerations  respecting  the 
physical  causes  which  enable  light  to  impart  to  an  organ  of  vision  the  phenomena  of 
tints,  but  to  show  that  the  idea  of  coloration  must  be  admitted  for  the  chemical  rays. 

334.  As,  in  the  foregoing  chapter,  we  gathered  our  final  views  respecting  absorption 
from  considerations  originally  drawn  from  one  case;  so,  in  this,  let  us  examine  the 
phenomena  exhibited  in  one  or  two  cases,  and  then  generalize  from  them.  We  have 
already  seen  (Ap.,  597),  that  when  a  ray  has  impinged  on  a  sensitive  surface,  as  on  a 
Daguerreotype  plate,  and  been  reflected  by  it,  it  has  lost,  to  a  great  extent,  the  power 
of  again  producing  the  same  effect,  or  is  detithonized. 

335.  The  remarkable  process  discovered  by  Sir  John  Herschel,  and  called  by  him 
chrysotype,  enables  us  to  verify  in  an  easy  and  very  satisfactory  way  the  truth  of  this 
remark. 

336.  Case  of  the  Chrysotype. — The  sensitive  material  employed  is  the  ammonio- 
citrate  of  iron;  a  ^o/?//^^??,  which,  when  viewed  through  small  thicknesses,  is  of  a  yellow 
colour.    From  its  being  a  solution,  it  is  peculiarly  fitted  for  these  experiments. 

337.  When  a  piece  of  paper,  washed  over  with  this  yellow  solution,  is  exposed  to 
the  sun  behind  a  trough  containing  the  same  solution,  the  paper  is  found  to  change 
very  slowly,  showing  that  the  liquid  in  the  trough  is  absorbing  the  active  rays. 

338.  Prismatic  Analysis  of  the  Chrysotype. — (a.)  I  projected  a  motionless  spectrum 
on  chrysotype  paper,  and  speedily  obtained  an  impression  of  a  pale  brown  colour,  which, 
when  brought  out  by  neutral  chloride  of  gold,  was  found  to  extend  from  a  to  (i.  Jig. 
126,  X. 

(b.)  Having  passed  a  beam  from  the  heliostat,  through  a  trough  with  parallel  sides, 


86 


LAWS  DEDUCED  FROM  IT. 


containing  a  solution  of  ammonio-citrate  of  iron,  of  such  strength,  and  in  a  stratum  oi 
such  thickness,  as  to  appear  of  a  bright  yellow  colour,  I  dispersed  it  by  the  prism,  and 
received  the  spectrum,  as  before,  on  chrysotype  paper.  For  a  long  time  the  paper  re- 
mained unchanged,  but  after  an  hour's  exposure  I  was  able  to  bring  out  a  very  faint 
mark,  the  position  of  which  was  6,  e,  fig.  126,  x. 

339.  The  inference  which  plainly  arises  from  these  experiments  is,  that  the  active 
chrysotype  rays  are  absorbed  by  the  ammonio-citrate  of  iron;  or,  in  other  words,  that 
this  substance  is  sensitive,  because  it  absorbs  a  peculiar  class  of  rays.  No  change  can 
take  place  in  chrysotype  paper  by  rays  that  have  passed  through  ammonio-citrate  of 
iron,  because  they  have  been  absorbed,  and  are  already  expended  in  effecting  the  re- 
quired decomposition. 

340.  The  same  conclusion  was  arrived  at  by  experimenting  in  the  followmg  way : 
I  prepared  a  sensitive  plate  by  exposure  to  iodine  and  bromine  successively,  which 
gives,  as  is  well  known,  a  very  changeable  surface.  This  plate  may  be  called,  for  the 
sake  of  distinction,  a  test-plate. 

341.  On  this  test-plate  I  received  a  spectrum  formed  from  a  beam  which  had  passed 
through  the  trough  containing  ammonio-citrate  of  iron.  After  a  suitable  exposure,  I 
found  a  stain  reaching  from  y  to  a,  Jig.  126,  y.  But,  as  is  shown  in  Jig.  126,  x,  the 
rays  which  affect  the  ammonio-citrate  of  iron  reach  from  a  to  (3.  Consequently,  we 
perceive  that  those  which  affect  the  test-plate  are  complementary  to  those  which  affect 
the  chrysotype.    We  draw,  therefore,  these  farther  conclusions  : 

1st.  That  the  rays  ivhich  escape  ahsor^ition  hy  the  ammonio-citrate  oj  iron  are  pre- 
cisely those  ivhich  do  not  affect  it  chemically. 

2d.  That  the  rays  which  are  absorbed  by  the  ammonio-citrate  of  ii'on  are  the  rays 
which  produce  chemical  changes  in  it. 

342.  Let  us  take  a  second  case,  selecting  for  consideration  the  bichromate  of  potash. 
Case  of  the  Bichromate  of  Potash. — As  is  well  known,  a  piece  of  paper  dipped 

in  a  solution  of  this  salt  speedily  turns  brown  on  exposure  to  the  sun's  rays ;  but  if  there 
be  placed  before  it  a  trough  containing  a  solution  of  the  salt,  then  the  change  goes  on 
very  slowly. 

343.  Prismatic  Analysis  of  this  Case. — On  projecting  a  motionless  prismatic  spectrum 
on  this  paper,  an  impression  was  obtained  in  a  quarter  of  an  hour,  which  extended  from 
a  to  (i,Jig.  121,  X. 

344.  A  trough  with  parallel  faces,  filled  with  a  solution  of  the  salt,  was  next  inter- 
posed in  the  beam,  and  the  resulting  spectrum  received  on  a  bromoiodized  test-plate.  It 
extended  from  y  to  a,  fig.  127,  y. 

345.  In  reference  to  the  test-plate  used  in  these  cases,  its  applicability  depends  on 
a  fact  pointed  out  by  Sir  J.  Herschel  {Phil.  Tram.,  1840,  p.  38),  that  bromide  of  sil- 
ver is  equally  affected  by  all  the  rays  of  the  spectrum.  In  using  it  as  here  described, 
the  experimenter  must  assure  himself  that  sufficient  of  the  bromine  has  been  employed 
to  give  sensitiveness  to  the  extreme  rays  of  the  spectrum ;  it  should  produce  such  a  ti- 
thonograph  as  that  given  inj^g^.  128,  the  red  region  being  fully  brought  out. 

346.  On  examining        126,  .r,  126,  ?/,  we  perceive  that  they  prove  for  the  bichro- 


LAWS  DEDUCED  FROM  I'J'.  37 

mate  of  potash  what  has  heen  ah-eady  proved  for  the  chrjsotype  preparation — that  the 
active  rays  are  absorhed,  and  that  the  inactive  rays  escape. 

347.  Without  dwelhug  longer  on  the  detail  of  farther  instances,  it  appears  that  the 
general  laws  under  which  these  phenomena  take  place  are  as  follows : 

1st.  When  a  ray  impinges  on  a  sensitive  surface,  or  passes  through  a  changeable  me- 
dium, with  the  chemical  effect  that  takes  place,  the  constitution  of  the  ray  is  corre- 
spondingly disturbed.  A  change  in  the  composition  of  the  medium  involves  a  change 
in  the  ray. 

2d.  Rays  which  thus  disappear  by  absorption  are  occupied  in  disturbing  the  consti- 
tution of  the  ponderable  medium. 

3d.  Rays  which  are  inactive,  or  which  are  not  involved  in  the  chemical  change  go- 
ing on,  escape  from  the  medium  by  being  transmitted  or  reflected. 

348.  The  definite  views  which  w^e  thus  gather  respecting  the  absorption  of  the  dif- 
ferent constituents  of  the  solar  rays,  and  the  production  of  chemical  changes,  lead  us 
by  very  simple  steps  to  regard  one  as  the  cause  and  tlie  other  as  the  effect.  In  cases 
like  these,  the  safest  way  to  true  conclusions  is,  to  be  guided  by  analogies.  It  is  true, 
that  the  properties  of  the  different  agents  in  the  solar  beams  are  sufficiently  distinct, 
but,  as  radiant  principles,  they  have  certain  qualities  in  common.  The  heat  of  a  sun- 
beam converged  by  a  lens  on  red  oxide  of  lead  is  absorbed  by  that  substance ;  oxygen 
gas  is  given  off,  and  a  lemon-coloured  protoxide  remains  behind  ;  no  farther  absorp- 
tion of  heat  now  takes  place,  and  no  farther  chemical  changes  ensue.  Heat,  therefore, 
as  well  as  light,  or  the  tithonic  or  phosphoric  rays,  in  producing  its  effects,  undergoes 
absorption. 

349.  In  thus  making  the  phenomenon  of  absorption  the  fundamental  fact  of  our  the- 
ories on  the  chemical  action  of  the  sunbeam,  and  in  giving  a  distinct  prominence  to  it, 
a  great  deal  of  precision  will  be  brought  into  our  theoretical  discussions.  Almost  every 
experimenter  has,  to  a  certain  extent,  recognised  the  truth  of  these  views  in  a  general 
way,  though  without  clearly  setting  forth  the  exact  conditions  under  which  absorption 
takes  place.  In  1841  (Ap.,  Ch.  XII.)  I  published  some  experiments  to  show  how 
completely  all  the  chemical  changes  effected  by  light  were  under  the  control  of  ab- 
sorption, and  that  the  sensitiveness  of  any  given  changeable  compound  could  be  altered 
by  altering  its  optical  constitution. 

350.  So,  in  the  case  of  radiant  heat,  a  piece  of  polished  silver  exposed  to  the  focus 
of  the  most  powerful  burning  mirror  never  melts,  not  because  it  is  an  infusible  body, 
but  because  such  an  optical  constitution  has  been  given  to  it,  that  it  reflects  the  heat 
which  impinges  on  it.  If  the  polish  be  taken  off,  and  the  surface  slightly  roughened, 
it  melts  in  an  instant,  because  it  can  absorb  the  rays.  So,  too,  different  coloured  pieces 
of  cloth,  exposed  to  the  sunshine  upon  snow,  will  sink  to  different  depths,  because  the 
quality  of  coloration  which  they  possess  enables  them  to  absorb  the  heat  more  or  less 
rapidly,  and  the  calorific  effect  is  determined  by  the  optical  constitution. 

351.  Apparently  nothing  can  be  of  a  more  irregular  character  than  the  photographic 
impressions  and  points  of  maxima  left  by  the  solar  spectrum  on  various  surfaces.  With 
trivial  causes  the  position  and  dimensions  of  those  impressions  change,  but  when  we 


m 


APPLICATION  TO  SPECTRUM  STAINS. 


come  to  consider  their  mode  of  origin,  as  thus  connected  with  absorptive  influence, 
nothing  is  more  plain  or  easy  to  understand.  We  must  regard  them  as  phenomena 
of  exactly  an  equivalent  character  to  those  of  the  different  appearances  exhibited  by  the 
luminous  spectrum  when  it  is  received  on  variously-coloured  paper.  For,  in  respect 
of  the  tithonic  rays,  surfaces  that  have  the  same  tint  to  our  eyes  act  like  surfaces  of 
different  colours,  A  solar  spectrum  received  on  a  surface  of  lampblack  is  scarcely 
visible.  On  a  piece  of  red  paper  the  red  and  orange  rays  are  copiously  reflected,  the 
others  more  or  less  absorbed.  On  a  yellow  paper  the  yellow  and  orange  are  brilliantly 
given,  but  the  blues  almost  disappear ;  on  a  blue  surface  the  more  refrangible  rays  are 
brightest,  the  yellows  have  faded  away.  So,  when  sensitive  surfaces  are  exposed  to  the 
spectrum,  they  give  us  an  expression  of  their  particular  action.  Bromide  of  silver  ab- 
sorbs more  uniformly  than  any  other  body  that  we  know  rays  of  every  refrangibility. 
In  respect,  therefore,  of  the  tithonic  rays,  it  acts  as  lampblack  does  to  the  luminous, 
and  might  be  regarded  as  a  black  body.  Iodide  of  silver  absorbs  the  blue,  and  reflects 
the  red,  the  orange,  the  yellow,  and  part  of  the  green.  To  eyes,  therefore,  which  could 
perceive  those  invisible  rays  which  it  reflects,  it  would  be  seen  as  though  acting  as  a 
ruddy-coloured  body,  and  giving  forth  rays  like  those  which  nitrous  acid  gas  transmits. 

352.  When,  therefore,  we  find  on  a  sensitive  surface  which  has  been  exposed  to  the 
spectrum  a  given  stain,  we  infer  that  rays  corresponding  in  refrangibility  to  the  place 
of  the  stain  have  been  absorbed,  and  the  rest  reflected  or  transmitted.  And  if  this  be 
true,  our  views  will  be  greatly  facilitated  if  we  resort  to  some  simple  method  of  nomen- 
clature, which  shall  be  descriptive  of  the  facts  observed.  It  is  for  this  reason  that  we 
have  proposed  to  recognise  the  phenomenon  of  coloration  for  these  dark  rays,  and  speak 
of  red  tithonic,  yellow  tithonic,  or  blue  tithonic  rays,  as  pointing  out  in  a  general  man- 
ner the  place  in  the  spectrum  of  the  ray,  the  properties  of  which  we  are  discussing. 
Extending  these  ideas  to  the  physical  characters  of  ponderable  bodies,  we  would  assert, 
with  Melloni,  that  they  have  an  invisible  coloration  of  their  own ;  that  bromide  of  sil- 
ver is  tithonic  black,  though  as  respects  hght  it  is  white  ;  that  iodide  of  silver  is  tithonic 
red,  though  as  respects  light  it  is  of  a  lemon  yellow. 

353.  Aided  by  these  definite  views  of  absorptive  action,  and  the  concomitant  phe- 
nomena of  coloration,  it  becomes  interesting  to  examine  whether  these  principles  are 
applicable  to  cases  in  which  chemical  changes  are  brought  about  by  the  action  of  light. 
It  is  with  a  view  of  showing  that  all  these  things  hold  in  the  decomposition  of  carbonic 
acid  by  leaves  under  the  light  of  the  sun,  that  we  have  entered  on  this  minute  dis- 
cussion. 

.  354.  The  law  under  which  the  discharge  of  vegetable  colours  in  the  solar  spectrum 
takes  place  has  not  escaped  the  penetration  of  Sir  J.  Herschel,  who  has  furnished  us 
with  so  much  that  is  new  in  this  department  of  science.  "  The  rays  effective  in 
destroying  a  given  tint  are  in  a  great  many  cases  those  whose  union  produces  a  colour 
complementary  to  the  tint  destroyed,  or,  at  least,  one  belonging  to  that  class  of  colours 
to  which  such  complementary  tint  may  be  referred." — (Phil.  Trans.,  1842,  p.  189.) 

355.  Now  this  is  nothing  more  than  an  expression  of  a  particular  case  of  absorptive 
decomposition,  in  which  light  is  the  agent,  and  vegetable  matter  the  substance  involved. 


/ 

HERSCHEL'    LAW  FOR  LIGHT.  89 

The  reason  that  a  yellow  substance  is  bleached  by  blue  rays,  is  because  it  absorbs 
those  rays,  for  the  very  same  reason,  therefore,  that  it  looks  yellow.  A  purple  vegeta- 
ble body  is  bleached  by  the  yellow  and  green  rays,  aiid  because  it  absorbs  those  rays  it 
looks  purple. 

356.  As  respects  light,  the  phenomena  of  coloration  are  obvious  to  our  organs  of  vision ; 
as  respects  the  dark  beams  of  heat,  of  chemical  action,  and  of  phos})horescence,  they 
must  be  hypothetical  or  ideal ;  but,  in  the  same  manner  that  Melloni  has  found  the 
admission  of  them  for  the  calorific  rays  of  such  admirable  advantage,  so,  in  this  depart- 
ment of  science,  similar  and  palpable  advantages  arise.  Who  could  for  a  moment 
doubt  that  light  and  the  tithonic  rays  were  agents  totally  distinct,  so  soon  as  he  came 
to  understand  that  to  one  of  them  iodide  of  silver  is  yellow,  to  the  other  orange  ;  to 
one  chloride  of  silver  is  white,  to  the  other  red ;  to  one  bromide  of  silver  is  white,  to 
the  other  it  is  black  \ 

357.  These  principles  undergo  a  severe  test  when  we  examine  the  phenomena  that 
arise  when  sensitive  surfaces  of  different  degrees  of  thinness  are  used.  As  is  we!! 
known,  if  a  silver  plate  be  exposed  to  the  vapours  of  iodine,  it  passes  through  several 
orders  of  colour,  red,  yellow,  blue — red,  yellow,  blue,  &c.,  &c.  In  each  one  ol  these 
series,  as  I  formerly  showed,  the  yellows  are  chemically  the  most  sensitive  (Ap.,  622,  &c.). 

358.  If,  therefore,  we  successively  expose  to  the  solar  spectrum  thin  plates  of  iodide 
of  silver  of  the  above-named  tints,  we  might  infer  that  the  resulting  impression  should 
change  its  position  with  the  colour  of  the  plate ;  that,  if  that  colour  was  yellow,  the 
rays  corresponding  to  the  blue  should  be  absorbed,  and  the  spectrum  impression  be 
found  among  the  more  refrangible  rays ;  that,  if  the  colour  was  blue,  a  tint  arising  from 
absorption  of  the  yellow,  a  spectrum  stain  should  be  found  corresponding  to  the  posi- 
tion of  the  yellow  rays,  and  not  to  the  blue,  these  last  undergoing  reflexion ;  and, 
finally,  as  the  colour  of  the  plate  changed,  so  should  the  photographic  spectrum  shift 
its  position. 

359.  But,  on  making  the  experiment,  I  found  that  this  result  does  not  arise ;  it  is 
immaterial  what  the  colour  of  a  Daguerreotype  plate  may  be,  the  spectrum  leaves  upon 
it  an  impression  in  an  invariable  position.  In  the  absence  of  diffused  light,  this  impres- 
sion is  entirely  among  the  more  refrangible  rays. 

360.  But  as  the  colour  of  the  plate  changes,  although  the  photographic  impression 
remains  invariable  in  position,  it  undergoes  variations  in  intensity.  It  exhibits  the 
deepest  stain  when  the  plate  is  yellow,  is  more  faint  when  the  plate  is  red,  and  be- 
comes hardly  perceptible  when  the  plate  is  of  a  grayish  metallic  aspect.  Time,  of 
course,  enters  as  an  element  into  these  results ;  a  gray  colour  will  receive  as  deep  an 
impression  as  a  yellow,  if  the  period  of  exposure  be  inordinately  prolonged. 

361.  From  this  we  gather,  that  on  exposing  films  of  iodide  of  silver  of  different 
thicknesses,  and  therefore  of  different  colours,  to  the  spectrum,  the  resulting  impression 
does  not  shitt  its  place,  but,  remaining  fixed  in  position,  undergoes  variations  in  sensi- 
tiveness— variations  which  are  exhibited  by  differences  in  the  depth  of  the  stains.  And 
this  result  is  a  striking  instance  of  the  doctrine  of  ideal  coloration.  Upon  the  princi- 
ples of  that  doctrine,  it  meets  with  a  beautiful  explanation. 

M 


90 


MODE  OF  ACTION  OF  THE  TITHONIC  RAYS, 


362.  The  iodide  of  silver  is  a  body  wliicli  copiously  absorbs  blue  tithonic  rays,  it 
quality  arising  from  its  chemical  relations  and  constitution.  It  is  unaffected  by  the 
pure  yellow  and  red  tithonic  rays,  when  they  operate  alone,  and  diffused  light  is  exclu- 
ded. Consequently,  it  can  give  no  indications  dependant  on  the  presence  or  absence 
of  them.  The  sunlight  to  it  is  monochromatic,  or  nearly  so,  for  it  is  decomposed  by 
blue,  indigo,  and  violet  rays  only.  The  phenomena,  therefore,  which  it  will  exhibit 
when  in  thin  plates,  are  such  as  would  be  exhibited  by  thin  transparent  plates  of  air, 
or  water,  or  glass,  on  which  a  monochromatic  light  is  falling.  If  monochron)atic  light 
falls  upon  plates  of  glass  of  variable  thickness,  the  reflected  beam  simply  changes  in 
intensity,  and,  if  the  language  of  science  permitted  us  to  describe  bodies  in  their  opti- 
cal relations  as  being  more  sensitive  to  light  when  they  failed  to  reflect  it  to  our  eye, 
and  less  sensitive  the  more  copious  the  reflexion,  we  can  understand  that  the  plate  of 
glass  would  pass  -through  all  orders  of  sensitiveness  as  its  thickness  varied.  At  one 
time  it  would  reflect  all  the  incident  beam,  and  as  it  increased  in  thickness  the  ray 
would  diminish  in  intensity,  and  finally  disappear,  and,  with  a  still  farther  increase,  the 
brilliancy  of  the  beam  would  again  be  reassumed,  and  so  on  through  successive  periods. 

363.  N^ow  this  is  absolutely  the  same  phenomenon  as  that  exhibited  by  iodide  of  silver 
in  films  of  variable  thickness  when  exposed  to  the  spectrum.  It  depend  on  the  ideal  col- 
oration of  the  iodide. 

364.  Let  us  examine,  in  the  next  place,  what  should  be  the  event  when  bromide  of 
silver  is  used  instead  of  iodide. 

In  order  to  enable  us  to  predict  the  result,  we  have  simply  to  consider  what  would 
occur  if  a  film  of  lampblack,  or  of  any  other  perfectly  absorbent  body,  could  be  obtain- 
ed, of  suitable  thinness,  and  exposed  to  the  luminous  spectrum.  For,  to  use  a  some- 
what objectionable,  but  perhaps  emphatic  expression,  bromide  of  silver  is  the  lamp- 
black of  the  tithonic  rays.  It  is  obvious  that  a  perfectly  absorbent  film  would  uni- 
formly appear  black,  no  matter  what  its  thickness  might  be.  The  tints  of  thin  plates 
arising  in  interferences  among  reflected  or  transmitted  rays  must,  in  this  instance,  be 
absent.  A  film  of  bromide  of  silver  must,  therefore,  have  a  uniform  sensitiveness — a 
sensitiveness  which  is  independent  of  its  thickness. 

365.  Having  thus  explained  the  laws  of  absorption  and  the  doctrine  of  ideal  color- 
ation in  this  and  the  preceding  chapter,  it  remains  only  to  add  a  few  words  on  the 
mode  of  action  of  the  tithonic  rays. 

366.  There  is  no  reason  to  believe  that  oxygen,  hydrogen,  or  nitrogen  gases,  in 
masses  of  ordinary  magnitude,  exert  any  perceptible  absorptive  effect  on  light,  heat,  or 
the  tithonic  rays.  These  bodies,  therefore,  and  all  others  having  the  same  relation, 
can  exert  no  action  on  each  other,  even  though  they  are  under  the  influence  of  the 
most  intense  radiation. 

367.  A  mixture  of  oxygen  and  hydrogen  gases,  exposed  to  a  brilliant  light,  can  never 
produce  water,  because  neither  of  its  constituents  has  the  power  of  absorbing  the  in- 
cident rays. 

368.  But  a  mixture  of  chlorine  and  hydrogen  gas  explodes  in  an  instant  under  the 
influence  of  light,  because  the  chlorine  can  exert  a  powerful  absorbent  action. 


CONNEXION  BETWEEN  ABSORPTION  AND  CHEMICAL  ACTION.  gi 

369.  From  these  theoretical  considerations  it  would  appear  that,  in  the  case  of  chlo- 
rine and  hydrogen,  the  latter  gas  is  wholly  passive ;  the  chlorine,  heing  acted  upon, 
absorbing  the  chemical  rays,  is  thrown  into  such  a  condition  that  union  takes  place 
with  the  hydrogen. 

370.  A  remarkable  consequence  follows  from  these  views.  If  the  reason  that  oxy- 
gen and  hydrogen  cannot  form  water  under  the  influence  of  the  sunlight  be  due  to  the 
circumstance  that  neither  of  those  gases  can  absorb  tithonic  rays,  but  are  perfectly 
transparent  and  colourless,  and  the  reason  that  chlorine  and  hydrogen  at  once  form  mu- 
riatic acid,  be  due  to  the  absorbent  capacity  of  the  chlorine,  it  results  that  when  a  mix- 
ture of  these  latter  gases  intercepts  a  ray,  the  absorbent  action  upon  that  ray  should 
not  be  greater  than  that  of  the  chlorine  alone,  and  even  not  more  than  one  half,  because 
of  the  diluted  state  in  which  the  chlorine  is  presented.  But  this  is  the  same  conclu- 
sion to  which  we  have  previously  arrived  by  direct  experiment  (321). 

371.  The  views  which  have  been  given  in  this  chapter  serve  to  show  that  chemical 
action  is  the  uniform  result  of  absorption  ;  but  the  converse  of  the  proposition  doe^  not 
hold  good ;  absorption  is  not  necessarily  attended  by  chemical  action.  Nevertheless, 
it  is  attended  with  a  certain  effect.  Even  in  the  case  of  an  elementary,  and  therefore 
unchangeable  substance  like  chlorine,  a  disposition  or  capacity  for  union  is  communi- 
cated. Chlorine  that  has  been  exposed  to  the  sun  unites  with  hydrogen  more  readily 
than  chlorine  which  has  been  made  and  kept  in  the  dark  (Ap.,  Ch.  XVIII.), 


CHAPTER  XL 

ON  THE  MODE  OF  ACTION  OF  LIGHT  IN  DIRECTING  THE  DIGESTION  OF  PLANTS. 

Contents  :  Connexion  hetiveen  Absorption  and  Chemical  Action. — Radiant  Matter  is 
absorbed  in  producing  different  Effects. — Reappearance  of  the  Force  expended. — ■ 
haws  of  Preliminary  Absorption  and  Definite  Action  observed  by  Plants. — Increased 
Rapidity  of  Vegetation  implies  increased  Brilliancy  of  the  Incident  Light. —  The 
Sun  probably  a  Periodic  Star. — Description  of  the  Mode  of  Action  of  Light  and 
Radiant  Heat  on  Leaves. 

372.  We  are  now  ready  to  take  up  the  consideration  of  the  question  proposed  in 
Chapter  II. :  In  what  manner  does  light  act  in  directing  the  digestive  function  of 
plants  ? 

373.  From  various  phenomena  exhibited  by  radiant  heat,  M.  Melloni  has  developed 
the  doctrine  of  invisible  calorific  coloration,  those  of  latent  heat  and  absorption  having 
been  established  many  years  ago.  In  the  same  manner,  from  phenomena  connected 
with  the  tithonic  rays,  we  have  developed  for  them  analogous  doctrines  in  the  prece- 
ding chapters.  As  respects  light,  the  views  which,  for  the  other  imponderables,  are 
only  ideal  or  imaginary,  for  it  become  certain;  because  our  organs  of  vision  inform  us 


92 


RADIANT  MATTER  IS  ABSORBED. 


at  once.  A  ray  of  liglit,  dispersed  by  the  action  of  a  prism,  presents  to  us  a  spectrum  in 
"which  we  see  plainly  intrinsic  differences  of  the  various  parts.  Exposed  to  this  spec- 
trum, or  to  these  coloured  spaces,  vegetable  juices  of  different  tints  undergo  modihca- 
tions;  some  are  changed  by  the  blue,  some  by  the  yellow,  some  by  the  red  ray.  If  we 
examine  the  conditions  under  which  these  things  take  place,  as  we  have  done  for  heat 
ami  for  tithonicity,  we  find  similar  evidences  of  absorption.  Thus,  chloropliyl,  which 
gives  a  green  colour  to  leaves,  undergoes  a  change  by  light,  and  becomes  wiiite ;  and 
when,  by  prismatic  analysis,  we  inquire  what  rays  are  active  in  producing  this  effect,  we 
find  that  they  are  those  which  have  been  absorbed. 

374.  For  plants,  therefore,  coloration  is  intimately  connected  with  chemical  action. 
There  is  a  chemical  reason  why  leaves  are  green  and  flowers  are  never  black,  but  un- 
fold a  painted  corolla  ;  and,  guided  by  these  principles,  we  can  see  that  all  those  various 
active  principles  which  are  stored  up,  all  the  different  juices  which  circulate,  are  con- 
nected with,  and  have  borne  a  certain  relation  to  the  coloured  portions.  In  a  rose,  the 
leaves  of  which  are  green,  and  petals  red,  and  interior  organs  of  reproduction  of  a  gaudy 
yellow,  these  tints  do  not  result  from  a  wanton  play  of  Nature,  but  are  rather  evidences 
of  premeditation  and  forethought.  As  anatomists  have  been  enabled  to  deduce  from 
their  studies  a  testimony  for  the  existence  of  a  Universal  Designer,  so,  too,  might  chem- 
ists, from  reasons  on  the  colours  of  a  wild  flower,  show  that  those  colours  are  the  means 
of  accomplishing  certain  ends. 

375.  These  things  hold,  whatever  doctrines  we  assume  respecting  the  nature  of  light; 
whether,  with  Newton,  we  regard  it  as  consisting  of  emanations  of  particles  which  are 
exceedingly  small,  or,  in  the  more  refined  views  of  modern  philosophers,  as  undulations 
of  an  elastic  medium.  The  waves  of  sound  which  pass  through  our  atmosphere  in 
like  manner  produce  striking  and  permanent  results,  and  even  are  often  connected  with 
those  higher  philosophical  events  which  not  only  belong  to  material  things,  but  also  to 
the  world  of  intellectuality.  There  are  strains  of  music  which  have  been  listened  to 
in  youth,  and  have  communicated  a  permanent  impression  to  the  brain,  which,  in 
after  life,  spontaneously  present  themselves  to  the  memory.  It  is  true  that  these  things 
originate  in  intellectual  operations,  but  it  is  equally  true  that  the  channels  of  commu- 
nication through  which  they  have  passed  from  mind  to  mind  belong  to  the  inorganic 
world.  It  is  atmospheric  pulsations  which  are  thus  registered  in  the  brain.  In  that 
wonderful  organ  they  are  stored  up,  and  amid  the  hourly  change  of  every  part  of  the 
living  system,  the  constant  introduction  of  new  particles,  the  passing  away  of  those 
which  are  dying  or  effete,  these  aerial  pictures  are  permanently  preserved.  In  an  in- 
stant, and  spontaneously,  there  flashes  across  the  memory  a  recollection  of  events  which 
transpired  half  a  century  before,  and  which  have  been  buried  in  oblivion.  So,  also, 
with  the  undulations  of  light.  The  effect  of  these  in  no  case  passes  away,  but  leaves 
its  permanent  impression  on  material  things,  and  these  impressions,  though  for  many 
ages  they  may  lie  dormant,  reappear  again  at  their  proper  time,  and  produce  their 
proper  effect.  Ten  thousand  centuries  ago  the  sunbeams  fell  on  the  leaves  of  trees, 
and  decomposed  carbonic  acid  just  as  they  do  now ;  and  the  woody  matter  they  pro- 
duced was  buried  in  the  earth.    In  natural  affairs  no  such  thing  as  a  system  of  expe- 


REAPPEARANCE  OF  THE  FORCE  EXPENDED.  93 

dients  is  known,  but  all  results  from  the  operation  of  far-reaching  and  immutable  laws. 
In  those  remote  times  events  were  taking  place,  the  application  of  which  referred  not 
to  things  then  existing,  but  to  an  hereafter.  The  earth,  and  the  sea,  and  the  air  were 
enlivened  and  invigorated  by  the  sunbeams  as  they  are  now,  but  while  present  pur- 
poses were  subserved,  the  future  also  was  not  forgotten.  In  the  twihght  that  then  ex- 
isted, as  it  does  now,  the  wolf  followed  his  flying  prey,  and  by  the  light  of  the  same 
stars  the  royal  tiger  pursued  his  midnight  maraudings.  Nor  is  there  anything  op- 
posed to  the  order  of  Nature  in  those  things.  Intellectual  development  can  only  take 
place  when  a  thousand  natural  conditions  conspire.  Reason  and  analogy  would  equally 
lead  us  to  suppose  that,  in  a  majority  of  those  globes  which  are  scattered  through  the 
regions  of  space,  those  conditions  are  not  attained ;  that  they  are,  as  this  earth  was  for 
countless  ages,  a  dungeon  of  pestiferous  exhalations,  and  a  den  of  wild  beasts.  And 
yet  the  formative  forces  of  Nature  ar.e  at  work ;  the  plastic  fingers  of  light,  each  colour 
producing  its  proper  effect,  are  arranging,  and  decomposing,  and  modelling,  and  the 
work  which  thus  goes  on  in  silence  attains  perfection  in  the  lapse  of  ages.  In  those 
planets,  as  in  this,  the  atmosphere  and  the  sea  are  finally  brought  to  a  proper  constitu- 
tion, and  wild  animals  make  their  appearance,  and  then  intellectual  beings.  For  these, 
whatever  is  wanted  has  been  provided.  And  results  which  for  so  long  a  time  have  lain 
dormant  now  come  into  use,  just  as  those  forces  which  in  primeval  ages  were  expended 
by  light  in  the  reduction  of  carbon  by  leaves;  carbon  which  has  been  buried  in  the 
earth,  in  our  times  reappears  again,  and  is,  perhaps,  occupied  in  driving  the  steam-ship 
that  carries  these  pages  over  the  waves  of  the  Atlantic  Ocean. 

376.  Two  prominent  facts  must  always  be  present  to  the  mind  of  a  natural  philoso- 
pher— the  indestructibility  of  matter,  and  the  indestructibility  of  forces.  Disappearances 
of  the  one  or  of  the  other  are  only  fictitious  deceptions.  Throughout  the  universe  the 
quantity  of  matter  and  the  quantity  of  force  remains  forever  unchanged.  Material 
atoms  migrate  from  one  condition  to  another,  now  putting  on  the  form  of  a  solid, 
now  of  a  liquid,  now  of  a  gas;  so  also  with  forces,  which  are  occupied  in  producing 
sometimes  one,  and  sometimes  another  effect;  but  the  quality  of  the  former  and  the 
value  of  the  latter  remains,  under  all  circumstances,  unchanged.  And  when  such  con- 
siderations as  those  which  are  before  us  show  that  rays  of  light  of  different  colours 
have  certain  offices  to  discharge,  and  certain  chemical  effects  to  produce,  affinities  of 
given  intensities  to  overcome,  molecules  to  group  in  determinate  positions,  these  are 
things  which  can  only  be  done  by  the  expenditure  of  a  certain  force ;  but,  though  that 
force  be  expended,  it  is  not  destroyed  ;  it  is  ready  to  reappear  from  its  condition  of  trans- 
mutation. In  the  same  way  that,  all  over  the  American  Continent,  at  a  certain  period 
after  the  fall  of  the  leaf  the  Indian  summer  sets  in,  and  in  the  cold  weather  of  winter 
restores  the  warm  days  of  July,  the  heat  which  is  evolved  from  decaying  leaves  in  the 
forests  being  derived  from  the  sunrays  of  the  preceding  summer,  so  also  every  ray 
which  has  been  expended,  no  matter  of  what  colour  it  may  be,  or  what  has  been  the 
chemical  or  physical  result  with  which  it  has  been  connected,  or  what  series  of  trans- 
mutations it  has  passed  through,  is  ready  to  be  restored  in  its  pristine  energy,  and  give 
rise  to  its  equivalent  mechanical  effect.    And  hence  we  can  see  that  although,  so  far 


94 


PRELIMINARY  ABSORPTION  AND  DEFINITE  ACTION  OBSERVED  BY  PLANTS. 


as  the  whole  universe  is  concerned,  the  amount  of  force  never  varies,  there  are  peri- 
odical variations  in  its  distribution.  The  solar  rajs  which  year  after  year  are  impin- 
ging on  our  earth,  are  so  much  taken  from  him,  and  so  much  given  to  us ;  and,  judging 
from  these  things,  we  should  infer  that  the  amount  of  force  exhibited  on  the  surface  of 
our  globe  should  steadily  be  on  the  increase.  Are  not  these  also  the  ideas  which  wc 
conceive  from  geological  investigations  1  And  is  it  not  this  which,  from  a  silent  and 
tenantless  waste,  has  made  that  surface  the  abode  of  myriads  of  living  things  1  that 
produces  all  over  it  locomotion  and  activity,  and  that  makes  all  the  diiference  between 
what  our  earth  was  at  the  beginning  of  the  secondary  epoch,  and  what  she  is  now? 

377.  It  was  an  observation  of  the  older  botanists,  that  the  green  parts  of  plants  only 
possess  the  quality  of  reducing  carbon  from  the  air.  In  other  portions,  and  under  cer- 
tain circumstances,  a  reverse  action  takes  place,  and  carbon,  probably  under  the  form 
of  sugar,  is  oxydized.  This  never  takes  place  in  the  presence  of  chlorophyl.  The 
analogies  which  we  have  traced  between  the  mode  of  action  of  light,  heat,  and  the 
tithonic  rays,  would  serve  to  indicate,  that  in  this  remarkable  decomposition  the  lumi- 
nous rays  undergo  the  same  changes,  and,  indeed,  act  in  a  similar  way  to  radiant  heat 
w^hen  it  gives  rise  to  decompositions,  or  the  tithonic  rays  when  they  produce  surface 
alterations.  From  these  analogies,  we  should  judge  that  the  active  rays  in  this  instance 
undergo  a  true  absorption,  which  is,  perhaps,  divided  into  two  periods,  as  we  have  seen 
is  the  case  when  chlorine  and  hydrogen  unite  (317),  or  iodide  of  silver  is  decomposed. 
That  the  preliminary  absorption  is  observed,  numerous  facts  seem  to  indicate.  When 
leaves,  immersed  in  water  containing  carbonic  acid  in  solution,  are  set  in  the  sunshine, 
they  do  not  all  at  once  commence  evolving  gas,  but  a  certain  period  of  time  elapses  be- 
fore the  bubbles  pass  off  with  any  rapidity,  and  that  period  once  over,  they  seem  to  fol- 
low the  fluctuation  of  light  with  considerable  accuracy.  A  cloud  passing  before  the 
sun  restrains  the  speed  of  reduction,  and  an  increased  brilliancy  of  light  is  followed 
by  an  increasing  rate  of  decomposition.  From  the  mode  in  which  these  experiments 
are  necessarily  made,  there  is  not  a  perfectly  clear  proof  of  the  preliminary  absorption, 
for  it  is  not  impossible  that  the  hesitation  in  evolving  gas  which  is  observed  comes  from 
disturbing  causes.  It  may  require  a  certain  time  for  the  carbonated  water  to  find  its 
way  through  the  tissues  of  the  leaf,  and  to  reach  the  seat  of  action.  It  may  also  re- 
quire a  similar  period  of  time  for  the  evolved  gas  to  percolate  out.  But  still,  allowing 
for  these  disturbing  actions  as  much  time  as  might  be  reasonably  supposed  sufficient, 
there  can  be  little  doubt  that,  when  leaves  are  brought  out  of  the  dark  and  set  in  the 
sunshine,  a  certain  period  elapses  before  vigorous  action  sets  in — a  period  which  seems 
to  correspond  to  that  of  the  preliminary  absorption  observed  in  other  cases. 

378.  If  any  doubt  should  remain  on  that  point,  there  can  be  none  on  the  circum- 
stance that  light  observes  the  law  of  definite  action.  Not  only  is  this  apparent  from 
the  phenomenon  taking  place  with  a  rapidity  corresponding  to  great  increases  and  dim- 
inutions of  light,  but  also  that  minor  differences  are  rigidly  observed.  Even  in  the 
spectrum,  we  have  seen  that  the  rate  of  decomposition  follows  very  closely  the  order 
of  illuminating  power,  and  here  we  have  rays  of  various  refrangibility  and  of  different 
.colours  in  action.    If,  under  such  circumstances,  where  variations  of  colour  intervene, 


I 


CONTROL  OF  THE  BRILLIANCY  OF  LIGHT.  95 

the  course  of  the  phenomenon  seems  to  be  directed  by  mtrinsic  brilliancy,  there  can  he 
very  little  doubt  that,  for  the  same  colour,  or  for  white  light,  the  law  is  rigidly  observed, 
a  brighter  illumination  or  a  greater  quantity  of  light  producing  an  increased  effect,  a 
lesser  illumination  or  a  feebler  light  producing  a  diminished  effect. 

379.  It  seems  to  be  sitbstantiated  by  geological  facts,  that  in  former  ages  the  rapidity 
of  vegetable  growth  was  far  greater  than  it  is  now  ;  that  certain  plants,  which  with 
us  attain  only  to  an  insignificant  size,  in  those  times  reached  a  very  great  magnitude. 
So  ferns,  which  in  our  latitudes  are  now  of  an  insignificant  growth,  in  the  same  places 
were  formerly  evolved  into  trees.  The  general  character  of  vegetation,  also,  in  given 
latitudes,  points  to  a  former  period  of  greater  luxuriance — a  period  during  which  trop- 
ical trees  could  grow  in  the  temperate  zone.  Geologists  have  already  concluded,  from 
these  and  a  variety  of  other  observations,  that  the  surface  temperature  has  undergone 
a  diminution. 

330.  But  there  are  many  conditions  which  have  to  be  fulfilled  before  so  marked  a 
difference  could  have  taken  place.  The  rate  of  vegetable  growth  depends  on  many 
things  :  on  the  amount  of  aqueous  vapour  and  carbonic  acid  in  the  air,  on  the  mean  t(  m- 
perature  of  the  surface,  on  the  brilliancy  of  the  incident  light,  &c.  There  are  experi- 
ments which  seem  to  show  that  the  constitution  of  the  atmosphere  is  by  no  means  so 
favourable  as  it  might  be  ;  a  greater  quantity  of  carbonic  acid  in  it  is  attended  by  a 
greater  rapidity  of  growth,  and  since  those  times  of  which  we  are  speaking,  great  chan- 
ges have  taken  place  in  this  respect.  Of  the  carI)onic  acid  of  those  eras  much  is  now 
rtMnoved,  and  shut  up  in  the  earth  in  those  great  deposites  of  anthracite  and  bituminous 
coal  which  occur  on  an  extensive  scale  in  so  many  parts  of  the  world.  Large  quanti- 
ties, also,  unquestionably  derived  from  the  same  source,  now  form  an  integral  constitu- 
ent of  great  coral  reefs  and  limestone  rocks,  some  forming  mountain  ranges,  and  some 
in  the  sea.  It  can  admit  of  little  doubt  that,  since  those  times,  the  total  quantity  of 
organized  carbon  existing  on  the  earth's  surface,  and  constituting  the  parts  of  plants 
and  animals  conjointly,  has  been  on  the  increase — an  increase  attended-  by  a  diminu- 
tion of  the  quantity  unorganized  in  the  atmosphere.  M.  Dumas  has  well  observed, 
that  the  original  atmosphere  has  become  divided  into  three  parts  :  one  which  still,  in 
a  modified  Ibrm,  envelops  the  earth  on  all  sides,  constituting  its  present  atmosphere;  a 
second,  represented  by  the  aggregate  of  vegetal)les  and  animals  now  existing  on  the 
earth's  surface — for  plants  and  animals  are  nothing  but  condensed  air;  a  third,  envel- 
oped in  a  fossil  state  in  the  bowels  of  the  earth.  And  this  tri-partition  has  been  main- 
ly effected  by  the  agency  of  the  sun.  To  restore  things  to  their  primordial  condition, 
all  these  must  be  mingled  together  in  the  gaseous  form,  and  the  forces  that  have  been 
derived  from  the  solar  rays  restored  back  to  that  luminary  again. 

381.  But  an  excess  or  a  diminution  of  carbonic  acid  in  the  air,  provided  the  varia- 
tion is  within  certain  limits,  would  not  exert  an  exclusive  control  over  the  production 
ot  vegetable  organized  molecules.  Variations  of  temperature,  as  connnon  observation 
shows,  exert  a  very  great  effect ;  the  periodicity  in  the  seasons  and  processes  of  horti- 
culture are  sufficient  to  prove  this.  So  well,  indeed,  has  this  been  understood,  that  ge- 
.  ologists,  from  considerations  on  the  decline  of  vegetable  growth,  have  drawn  the  doc- 


9<5  THE  SUN  PROBABLY  A  PERIODIC  STAR. 

trine  of  diminution  of  terrestrial  temperature ;  a  doctrine  which  is  enforced  bj  argu- 
ments furnished  as  well  by  the  inorganic  world. 

382.  But  radiant  heat  is  not  the  primitive  force  which  organizes  the  carbon  atoms, 
and  groups  them  into  their  various  forms;  it  acts  a  subsidiary  part,  the  decomposition 
and  subsequent  arrangement  being  directed  by  light.  Whatever  facts,  therefore,  ex- 
ist, which  prove  an  increased  activity  in  vegetable  growth  in  tlie  early  times,  prove  also 
an  increased  brilliancy  in  the  light.  The  occurrence  of  an  excess  of  carbonic  acid  in 
the  air,  or  of  a  higher  temperature,  is  not  enough.  To  the  light,  which  is  the  vital 
agent,  a  greater  activity  must  be  assigned.  It  is  thus  we  perceive  that  changes  in  the 
interior  temperature  of  the  globe  can  have  had  only  an  indirect  connexion  with  what 
was  thus  going  on  on  its  surface.  And  if  there  have  been  periodic  vicissitudes,  if  plants 
have  once  grown  with  excessive  luxuriance,  and  in  short  spaces  of  time  withdrawn 
large  quantities  of  carbon  from  the  air,  this  is  a  result  which  is  connected  not  so  much 
with  internal  or  external  temperature  as  with  periodic  variations  in  the  brilliancy  of 
light. 

383.  Do  not,  therefore,  these  things  seem  to  indicate  that  our  sun  is  one  of  those 
periodic  stars,  the  light  of  which  undergoes  secular  changes ;  that  for  a  series  of  years 
or  of  centuries  it  increases  in  brilliancy,  and  then  fades  away  ;  and  that,  as  these  peri- 
ods pass  over,  corresponding  mutations  in  its  intensity  of  radiation  are  observed  1  That, 
affected  by  this,  the  rate  of  vegetable  growth,  the  character  of  animal  life,  the  constitu- 
tion of  the  atmosphere  is  simultaneously  changed  in  all  the  attendant  planets  ?  It  is 
of  no  consequence  to  say  that  great  and  almost  universal  mutations,  such  as  those  we 
are  here  describing,  are  not  consonant  to  the  ways  of  Nature ;  or  that,  in  the  periods 
of  human  history,  traces  of  such  operations  have  never  been  witnessed.  No  observa- 
tion in  philosophy  is  more  true,  than  that  "  changes  which  are  rare  in  time  become 
frequent  in  eternity." 

384.  But  among  the  stars  these  periodic  variations  do  take  place.  As  was  discov- 
ered by  Sir  J.  Herschel,  a  Orionis,  if  examined,  is  seen  to  increase  in  brilliancy  for 
several  days,  and  then  to  diminish.  In  the  same  way,  a  CassiojJeics  has  its  period  em- 
braced in  225  days.  And  many  other  instances  are  known.  It  signifies  nothing  that 
these  periods  are  short.  In  the  constitution  of  the  universe,  no  value  is  attached  to 
time.  With  men,  whose  period  of  action  is  embraced  in  a  few  years,  the  different 
events  of  life  are  circumscribed  by  measured  spaces — there  is  a  limit  beyond  which  hu- 
man exertion  cannot  go ;  and  to  adjust  time  and  action  to  each  other,  and  to  measure 
the  one  by  the  other,  is  our  common  duty.  But  in  the  administration  of  the  universe 
the  case  is  different ;  in  eternity  there  are  no  limits  of  duration,  and  time  can  be  ex- 
pended without  detriment  or  loss.  In  the  pulsation  of  a  wave  of  light,  a  part  of  the 
millionth  of  a  second  is  enough,  and  it  is  given.  In  the  revolution  of  one  system  of 
stars  round  another,  millions  of  centuries  are  required,  and  they  are  consumed.  And 
so,  in  the  case  that  we  are  considering,  the  glowing  and  fading  away  of  one  star  may  be 
accomplished  in  a  few  days,  but  inconceivably  great  periods  of  time  may  be  wanted  for 
the  same  events  to  transpire  in  another.  Even  philosophers  are  too  prone  to  believe, 
that  by  the  short  spaces  of  human  life,  or  the  history  of  nations,  they  can  mark  out  pe- 


I 


DESCRIPTION  OF  THE  ACTION  OF  LIGHT  ON  LEAVES.  97 

riods  in  eternity  ;  but,  whether  we  consider  the  scale  of  space,  or  of  time,  on  which  the 
univei'se  is  constructed,  we  can  see  that  our  minds  are  so  constituted  as  to  be  equally 
unable  to  appreciate  either  extremity  ;  that  we  can  attach  no  just  idea  to  what  is  either 
infinitely  great  or  infinitely  small ;  and  that  therefore  our  views  do  not  always  justly 
apply  in  natural  events.  Of  one  thing  we  may  rest  assured,  that  no  matter  how  great 
the  periods  that  may  be  required  for  the  phenomena  of  the  universe  to  transpire,  there 
has  been,  and  there  will  be  time  enough  for  their  endless  repetition. 

385.  From  considering  what  takes  place  when  a  green  leaf  is  enlightened  by  the 
sun,  we  are  thus  allured  to  pass  on  step  by  step  to  reflections  on  the  history  of  the 
solar  system,  and  to  changes  which  have  happened  to  the  earth.  In  the  same  way 
that  the  stem  of  an  exogenous  tree  is  a  lasting  record  and  memorial  of  the  returning 
summers  it  has  witnessed,  each  ring  that  we  see  pointing  out  the  growth  of  one  season, 
and  being,  so  to  speak,  an  index  of  the  amount  of  light  which  has  been  at  play ;  or, 
when  casting  our  eyes  over  the  climates  of  the  earth,  we  observe  in  the  tropics  a  rank 
vegetation,  and  trees  and  flowers  flourishing  all  the  year  round  ;  or,  coming  to  the  tem- 
perate zones,  we  find  a  hardier  growth,  and  the  soil  only  yielding  its  fruits  to  human 
industry  and  skill ;  or,  passing  towards  the  poles,  the  stunted  plants,  and  lichens,  and 
mosses,  and  great  plains  covered  with  perpetual  snow,  and  even  in  these  inclement  re- 
gions all  vital  operations  under  the  control  of  astronomical  causes.  Do  not  all  these 
serve  to  set  forth  the  entire  control  which  the  solar  rays  have  over  these  phenomena, 
and  teach  us  that  the  same  kind  of  reasoning  which  applies  to  things  taking  place  in 
our  time,  applies  also  to  things  which  have  preceded  it ;  that  if  the  section  of  an  ex- 
ogenous stem,  or  the  climate-distribution  of  plants,  point  out  a  present  connexion  and 
present  relations  with  the  sun,  so  do  those  fossils  which  are  dug  out  of  the  ground 
point  to  similar  relations  in  former  times;  their  magnitude  and  luxuriance  indicate  a 
more  brilliant  ray.  From  the  beginning  of  things  no  natural  law  has  ever  changed  ;  re- 
sults are  obtained  in  these  times  by  the  same  operations  or  mechanism  by  which  they 
were  obtained  of  old.  As  with  us,  so  then,  when  the  sunbeam  falls  on  a  leaf  the  yellow 
ray  is  absorbed,  and  if  carbonic  acid  is  present,  it  undergoes  decomposition,  green  mat- 
ter is  rapidly  formed,  a  mixture  of  oxygen  and  nitrogen  gases  is  emitted,  and  carbon, 
oxygen,  hydrogen,  and  nitrogen  are  fixed  in  the  plant.  The  seat  of  this  action  is  now, 
as  it  was  then,  that  face  of  the  leaf  which  is  exposed  to  the  sky;  and  the  nutritious 
juice  thus  formed  turning  over  to  the  under  face  of  the  leaf,  is  there  concentrated  by 
the  evaporatory  action  of  the  stomata.  From  these  chemical  changes  mechanical 
forces  arise,  and  the  nutritious  sap  is  impelled  downward,  or  brought  into  relation  with 
all  parts  of  the  plant.  From  it  sugar,  and  gum,  and  starch,  and  woody  fibre,  albumen, 
fibrine,  &c.,  are  formed,  and  these  are  lodged  in  various  parts,  or  stored  up  for  the 
farther  purposes  of  the  economy. 

386.  I  regard,  therefore,  the  sunlight,  when  acting  upon  plants,  as  operating  Exactly 
in  the  same  way  as  the  chemical  rays  or  radiant  heat,  when  they  produce  their  specific 
phenomena.  That,  first  of  all,  a  certain  absorption  takes  place,  which  seems  to  be  un- 
attended with  any  direct  effect,  and  of  the  nature  of  which  we  have  only  an  indistinct 
idea;  that  when  this  is  over,  the  rays  continuing  to  act,  and  the  tissue  of  the  leaves 

N 


DESCRIPTION  OF  THE  ACTION  OF  LIGHT  ON  LEAVES. 


being  filled  with  water  holding  carbonic  acid  in  solution,  part  of  which  acid  has  beeJv 
derived  from  the  air,  and  part  brought  through  the  spongioles  from  the  soil,  decompo- 
sition takes  place — a  decomposition  accomplished  under  the  law  of  the  definite  action 
of  light ;  tha:  H  the  rays  increase  in  brilliancy,  the  chemical  result  goes  on  with  more 
rapidity,  and  if  there  be  a  diminution,  the  chemical  result  correspondingly  declines; 
that  this  action  goes  forward  at  a  maximum  under  the  influence  of  the  yellow  ray,  the 
orange  and  the  green  coming  next  in  rank,  and  the  others  following  in  the  order  of  their 
illuminating  power ;  that  from  this  circumstance,  the  extreme  violet  and  extreme  red 
seem  to  possess  little  activity,  and  the  tithonic  rays  appear  to  be  in  no  manner  engaged, 
or  engaged  only  in  an  indirect  way,  after  the  same  manner  as  radiant  heat.  As  fast  as 
carbonic  acid,  dissolved  in  the  vegetable  juices,  is  disposed  of,  new  quantities  are  taken 
up,  some  little  coming  with  the  ascending  sap  from  the  ground,  but  the  great  part  being 
supplied  from  the  air;  for,  through  the  air,  by  diffusion,  gases  pass  with  great  rapidity, 
and  percolate  through  their  films  of  water  (Ap.,  80).  By  aerial  currents,  by  the  move- 
ment of  the  leaf  an  extensive  and  continually  renewed  contact  with  new  portions  of 
air  is  established ;  from  this  the  carbonic  acid  is  taken,  which  is  dissolved  in  the  watery 
juices  circulating.  Brought  under  the  luminous  influence,  it  undergoes  decomposition, 
its  carbon  and  a  portion  of  its  oxygen  being  appropriated  (Ap.,  818),  and  a  volume  of 
nitrogen  equal  to  the  volume  of  oxygen  thus  appropriated,  evolved  along  with  the  remain- 
ing oxygen.  There  is,  therefore,  a  removal  of  water  and  carbonic  acid  from  the  air;  a  fixa- 
tion of  carbon,  hydrogen,  oxygen,  and  nitrogen  in  the  plant,  and  an  evolution  to  the 
air  of  nitrogen  and  oxygen.  From  the  constant  appearance  of  the  former  of  these 
bodies,  we  are  led  to  suspect  that  the  light  acts  primarily  on  some  azotized  body,  the 
destruction  or  eremacausis  of  which  is  essential  to  the  total  action  (A  p.,  824);  while 
ail  this  is  going  on,  chlorophyl  is  abundantly  formed,  and  so  long  as  the  process  is  ac- 
complished, the  leaves  retain  their  green  colour. 

387.  On  several  occasions  it  has  been  said,  that  th  ■  phenomenon  here  described  an- 
s\\ers  to  a  true  digestive,  and  not  to  a  respiratory  process;  the  older  chemists  and  bot- 
anists confounded  it  with  the  latter  function  ;  but  it  is  obvious  that  it  does  not  answer 
to  respiration,  either  in  mode  of  operation  or  in  result.  Respiration  is  an  oxydizing 
process,  the  object  of  which  is  to  maintain  the  animal  machine  at  a  fixed  thermomet- 
ric  point — a  result  which  implies  direct  combustion  or  burning;  an  animal,  rigorously 
speaking,  burns  carbon  like  a  locomotive  engine.  But  in  this  action,  on  the  contrary, 
carbon  is  reduced  from  carbonic  acid,  and  there  should  be  a  descent  of  temperature  in- 
stead of  an  elevation,  a  large  amount  of  heat  being  absorbed — heat  which  is  furnished, 
under  natural  circumstances,  by  the  sun,  along  with  his  light.  The  continued  supply 
of  heat  in  this  way  prevents  us  from  discovering  that  reduction  of  temperature  which 
shonid  l)efall  the  leaf;  and,  besides  this,  it  is  exposing  to  the  open  atmosphere  its  broad 
suriace,  and  any  thermometric  disturbance  is  at  once  compensated  by  external  agen- 
cies. It  is  possible,  indeed,  that  the  decomposing  process  could  not  go  on,  save  under 
the  conjoint  presence  of  heat  and  light,  though  the  specific  function  which  each  of 
these  agents  discharge  may  be  different.  The  action  of  leaves  in  the  sunshine  bears, 
therefore,  no  sort  of  analogy,  either  in  manner  or  in  result,  to  the  respiratory  processes 


NERVOUS  AGENT  OF  PLANTS.  99 

of  animals,  no  matter  whether  the  mechanism  be  the  lungs  of  a  u)ammalian  or  the 
branchial  organs  of  fishes.  Every  variety  of  breathing  apparatus  has  for  its  object  the 
evolution  of  heat  by  the  oxidation  of  carbon,  or  of  carbon  and  hydrogen  conjointly  ; 
but  the  object  of  the  agency  of  leaves  upon  the  air  is  to  obtain  from  it  carbon,  or  car- 
bon and  hydrogen.  At  certain  periods  of  their  history,  plants  themselves  become  ma- 
chines of  combustion,  when  the  process  of  fertilization  requires  that  for  a  time,  in  a 
given  place,  and  for  a  specific  object,  there  should  be  an  elevation  of  temperature.  Re- 
sort is  then  had  to  those  same  processes  which  obtain  in  animal  systems  ;  and  sugar, 
or  what  comes  to  the  same  thing,  honey,  is  burned. 


CHAPTER  XII. 

ON  THE   NERVOUS  AGENT   OF  PLANTS. 

Contents  :  Subdivisions  of  Nervous  Mechanism  in  Animals. — Excessive  Rapidity  of 
Motion  arising  in  these  Nervous  Actions. — Plants  constructed  on  a  Surface-type. — Ox- 
idating Processes  replaced  in  them  hy  the  Application  of  Radiant  Heat. — Difference  of 
Action  on  the  Upper  and  Under  Face  of  the  Leaf. — Light  applied  to  one,  and  Heat  to 
the  other  Face. — Specific  Effects  produced  hy  the  different-coloured  Rays. — Effects  of 
these  Radiant  Principles  on  the  Lower  Tribes  of  Animals. — Centralization  of  Ajjjm- 
ratus  for  different  Functions. — Analogies  betiveen  Nervous  Action  in  Animals  and 
Imponderable  Agency  in  Plants. —  Vegetables  are  the  Representatives  of  the  Resultant 
Action  of  the  Ethereal  Agents  on  Ponderable  Matter. — Conclusion. 

388.  It  was  the  beautiful  discoveries  of  Sir  C.  Bell  which  first  forcibly  drew  the 
attention  of  physiologists  to  the  fact  that  different  portions  of  the  nervous  system  are 
devoted  to  different  functions ;  that  in  the  spinal  axis  there  is  one  column  devoted  to 
sensation,  and  another  to  motion.  This  division  of  offices  is  doubtless  carried  to  a  far 
greater  extent  than  we  have  at  present  any  means  of  proving.  Analogy  would  lead  us 
to  suppose  tha-t  every  function  is  represented  by  its  own  appropriate  mechanism.  It 
is  not  alone  in  the  great  and  more  striking  characteristic  divisions  of  action  that  these 
divisions  of  machinery  are  observed  ;  the  cerebrum,  the  cerebellum,  or  the  sympathetic 
system,  being  each  devoted  to  its  specific  end,  they  doubtless,  also,  exist  on  a  far  more 
minute  scale,  in  connexion  with  more  trivial  purposes. 

389.  If,  thus,  the  intellectual  processes  and  processes  of  movement,  which  are  things 
appertaining  to  the  interior  constitution  of  the  animal  system,  are  under  the  control  of 
a  divided  agency,  a  similar  plan  is  resorted  to  in  the  case  of  those  functions  which  put 
the  system  in  relation  and  conununication  with  the  exterior  world.  On  these,  the  out- 
ward physical  agents  have  to  expend  their  operation.  The  optic  nerve,  which  gath- 
ers on  its  retinal  expansion  the  images  of  outward  forms,  transmits  them  to  the  brain. 
To  that  cerebral  tract  to  which  it  goes,  the  power  is  given  to  be  affected  by  luminous 


100 


EXCESSIVE  RAPIDITY  OF  MOTIONS  ARISING 


agency  ;  it  is  immaterial  whether  that  agency  consist  of  undulations  of  an  ethereal  me- 
dium, or  spend  itself  in  producing  a  chemical  change  of  the  retina.  The  jwrtio  mollis 
of  the  seventh  pair,  also,  exposes  itself  in  the  cochlea  of  the  ear,  and  having  the  func- 
tion of  audition  committed  to  it,  vibrates  correspondingly  to  those  oscillatory  move- 
ments in  the  atmosphere  which  constitute  sound.  So,  too,  with  the  olfactory  nerve, 
which,  pushing  its  way  through  the  cribriform  plate  of  the  ethmoid  bone,  expands  in  a 
million  of  ramified  branches  on  the  Schniderian  membrane,  and  is  ready  to  be  impress- 
ed by  odours  or  smells.  There  is  no  such  thing  as  a  mumal  convertibility  of  the  offi- 
ces of  these  different  machines ;  no  vicarious  interchange  of  action ;  each  one  has  its 
own  duty  to  perform,  each  has  to  discharge  its  proper  task,  and  the  construction  of 
each  is  suitably  arranged.  In  human  contrivances,  the  same  necessity  of  result  arises ; 
the  telescope  will  not  answer  for  a  piano,  nor  the  piano  for  a  telescope. 

390.  Wliile  thus  the  different  senses  of  sight,  of  smell,  of  hearing,  of  taste,  and  of  touch, 
and  all  the  different  functions  which  occur  in  animal  frames,  are  carried  on  by  their  own 
appropriate  enginery,  and  resort  had  to  optic,  auditory,  olfactory,  and  respiratory  cords, 
these  several  contrivances  are  so  arranged  that  the  final  result  of  tlieir  operation  con- 
verges inward,  and  is  at  last  expended  on  the  same  point  from  which  also  spring  all 
the  various  acts  of  thought  and  intellectuality.  Of  that  central  point  which  is  thus  in 
incessant  agitation  during  the  continuance  of  animal  life,  perpetually  receiving  unpres- 
sions  from  outward  objects  or  the  external  world,  perpetually,  also,  reflecting  back 
again  the  various  determinations  of  the  mind,  how  rapid  and  how  constant  must  be  the 
movement !  We  cannot  comprehend  how  it  is  possible  that  an  ethereal  particle,  vibra- 
ting so  as  to  produce  violet  light,  oscillates  backward  and  forward  seven  hundred  and 
twenty-seven  millions  of  times  in  the  millionth  part  of  a  second;  yet  this  is  a  fact  as 
w^ell  established  as  any  other  fact  in  the  domain  of  science.  What,  then,  shall  we  say 
of  that  central  point  of  reception  which  is  within  the  brain,  which  stands  ready  to  ex- 
ecute at  once  all  the  synchronous  movements  impressed  upon  it  by  the  various  colours 
of  hght,  all  the  vibrations  of  simultaneously  occurring  and  harmonious  sounds,  all  the 
impressions  which  are  brought  to  it  by  the  apparatus  of  the  senses  !  During  the  time 
of  wakefulness,  how  is  it  agitated  by  these  various  movements !  and  during  the  time 
of  sleep,  its  activity  is  still  expressed  by  those  phantoms  which  we  see  in  dreams.  As, 
at  a  telegraphic  station,  the  observer  watches  the  various  signals  through  his  telescope, 
and  reports  to  his  government  the  intelligence  which  is  arriving,  so  that  central  vibra- 
ting point  reports  to  the  mind  the  telegraphic  despatches  that  are  coming  along  the  dif- 
ferent nerves.  Nor  is  it  impossible  that  one  material  atom,  or  even  a  small  congeries 
of  atoms,  should  be  able  to  be  thus  affected  at  once  in  a  thousand  different  ways  ;  a 
particle  of  water  on  the  surface  of  the  ocean  may  be  sinmltaneously  affected  by  mill- 
ions of  waves,  which  may  go  forth  from  it  and  separate  without  disturbing  the  move- 
ments of  each  other  ;  a  particle  of  ether  may  be  acted  upon  by  every  possible  ray  of 
light  that  can  reach  it — it  will  be  affected  by  the  general  action  of  all,  and  each  one 
will  go  forth  and  separate  from  all  the  others,  undisturbed  by,  and  undisturbing  them. 
But  even  were  it  not  a  single  physical  point — a  physical  point  in  our  ordinary  idea  of 
that  term — even  were  it  the  whole  brain  which  is  thus  agitated  and  acted  on,  what  but 


PLANTS  CONSTRUCTED  ON  A  SURFACE  TYPE.  101 

a  point  in  comparison  with  the  world  from  which  it  gathers  its  intelligence,  is  the  whole 
mass  of  the  brain  !  Is  it  not  affected  by  light  which  has  come  from  systems  of  suns  at 
the  uttermost  ends  of  the  universe  1  Does  it  not  waich  the  rotation  of  double  stars  of 
different  colours,  which  occupy  enormous  periods  of  time  to  complete  their  revolutions, 
and  are  situated  at  almost  immeasurable  distances  t 

391  Nervous  action  in  animals  is  thus  carried  on  by  a  con)plicated  mechanism,  and 
the  nervous  agent,  though  ordinarily  spoken  of  as  "one  and  indivisible,"  presents  itself 
to  our  view  as  the  reunion  of  many  forces,  or  as  one  force  with  many  modifications. 
To  the  inunaterial  and  responsible  principle  which  is  within  us,  it  stands  in  the  char- 
acter of  a  minister  and  messenger,  to  connect  it  with  the  outward  natural  world,  to 
gather  impressions  and  enable  it  to  react  on  material  things.  Hence  arises  that  sub- 
division of  functions  of  which  we  have  been  speaking,  a  subdivision  not  only  affecting 
external  impressions,  but  also  the  corresponding  individual  actions.  The  nervous  and 
optical  mechanism  of  the  eye  is  so  arranged  as  to  have  entire  charge  over  the  recep- 
tion of  impressions  conveyed  by  the  luminiferous  ether;  the  auditory  mechanism  of  the 
ear  is  constituted  so  as  to  receive  undulations  of  gaseous  bodies  like  atmospheric  air; 
and,  correspondingly,  if  intelligence  has  to  be  communicated  to  a  distance,  and  received 
by  other  minds  through  the  agency  of  a  visual  organ,  the  motor  nerves  of  the  hand  are 
put  in  action,  the  fingers  move,  and  letters  appear  upon  the  paper. 

392.  In  steam-engines  or  other  automatic  machines,  our  admiration  is  chiefly  aroused 
by  the  regular  and  consentaneous  manner  in  which  their  movements  are  performed. 
At  the  proper  instant  the  proper  valves  are  sprung,  and  the  intricate  motions  go  on 
with  regularity.  No  one  can  have  seen  a  large  cotton  or  silk  factory  in  which  the 
machinery  is  driven  by  one  of  these  engines  without  being  struck  with  this  remark. 
Perhaps  it  is  weaving  an  elaborate  pattern,  and,  with  a  discretion  that  almost  simulates 
intelligence,  is  putting  in  or  leaving  out  the  variously-coloured  threads.  To  produce 
this,  a  series  of  contrivances  is  made  to  intervene  between  the  point  of  action  and  the 
-motive  force,  and  accordingly  as  the  result  is  different,  so  must  that  intervening  mech- 
anism vary.  In  animal  systems,  the  same  observation  holds  good  :  injpressions  which 
come  to  us  from  the  external  world,  and  movements  which  result  from  mental  opera- 
tions, are  all  transmitted  by  their  proper  channels. 

393.  From  the  time  of  Aristotle,  it  has  been  observed  that  the  type  on  which  vege- 
tables have  been  constructed  bears  a  remarkable  relation  to  the  type  of  animals.  In 
the  latter  all  the  processes  of  organic  life,  such  as  digestion,  respiration,  secretion,  are 
carried  forward  in  interior  cavities ;  in  the  former  they  are  surface  actions.  Thus,  the 
stomach,  the  digestive  organ,  is  enclosed  in  an  interior  space,  but  the  leaf,  which  is  the 
analogous  organ  for  plants,  is  freely  exposed,  and  digestion  takes  place  upon  the  sur- 
face. So,  too,  with  respiration,  the  lungs  of  the  mammalia  are  placed  within  the  walls 
of  the  chest,  and  receiving  oxygen  from  the  air,  transmit  it  by  the  arterial  blood  to  the 
capillary  vessels,  in  which  the  combustion  of  carbon  is  going  on;  but  when  in  plants 
the  same  result  is  to  be  attained,  oxydation  takes  place  on  the  petals  of  the  corolla, 
which  is  another  surface  action.  In  animals  the  attempt  is  to  centralize  everything, 
;to  make  all  the  functions  subservient  to  the  perfect  development  of  one  focal  point,  to 


102 


OXYDATING  PROCESSES  REPLACED  BY  RADIANT  HEAT. 


which  all  the  nerves  of  sensation  go,  and  from  which  all  volitions  and  motions  arise. 
Ill  plants,  the  attempt  is  to  diffuse  everything,  to  have  no  centre  of  action,  but  to  exe- 
cute upon  the  periphery.  It  is  true  that  the  lower  order  of  animals  are  constructed 
on  this  type  of  diffusion,  and  it  is  interesting  to  see,  when  in  more  advanced  tribes  the 
attempt  at  centralization  begins,  how  rapidly  it  goes  on  to  perfect  development.  The 
appearance  of  an  isolated  organ  of  digestion — a  stomach — is  the  signal  of  an  isolated 
organ  of  circulation — a  heart — no  matter  though  that  heart  may  be  a  mere  tube,  as  the 
dorsal  vessel  of  many  insects. 

394.  If  thus,  in  animal  existence,  we  find  the  various  nervous  machines  divided  off, 
and  the  impressions  of  light,  of  sound,  of  taste,  committed  to  separate  apparatus,  how 
is  it  with  plants  1  The  rays  of  the  sun  are  the  true  nervous  principle  of  plants  !  And 
herein  we  see  how  closely  the  type  of  surface  action  is  observed.  On  all  sides  the 
leaves  present  their  thin  lamina  to  the  light,  and  offer  a  broad  surface  to  the  sky.  On 
these  the  rays  fall,  and  direct  the  digestive  and  other  processes.  When  they  are  car- 
ried on  in  interior  cavities,  and  the  living  system  assumes  a  certain  mass,  an  oxydating 
machine  is  demanded  in  order  to  keep  up  the  temperature  of  the  whole  by  the  burn- 
ing of  carbon,  and  lungs,  or  gills,  or  other  suitable  contrivances,  are  resorted  to.  But 
the  principle  on  which  the  vegetable  organization  is  constructed  is  simple,  and  except 
on  those  particular  occasions  to  which  reference  has  been  made  (387),  an  apparatus 
of  combustion  is  not  wanted.  For  with  the  rays  of  light  which  come  from  the  sun 
there  are  also  rays  of  heat,  and  these  impinging  on  the  outside  surface,  which  is  the 
seat  of  all  the  vegetable  activity,  are  absorbed,  and  bring  up  the  temperature  to  the  re- 
quired point.  What  need  is  there  for  the  burning  of  carbon  or  of  hydrogen,  when  on 
all  sides  radiant  heat  is  pouring  in  1  Nature,  with  a  provident  care,  has  in  all  ordinary 
leaves  provided  abundantly  for  this  elevation  of  temperature,  and  when  the  direct  beams 
do  not  reach  them,  warm  currents  of  air  that  rise  from  the  heated  ground  are  every- 
where present. 

395.  In  a  chemical  point  of  view,  it  is  the  quality  of  light  to  produce  the  organiza- 
tion of  molecules,  that  is,  to  determine  the  positions  of  atoms  as  respects  each  other; 
it  is  the  quality  of  heat  to  determine  their  distances ;  for  the  accumulation  of  caloric 
in  a  body  produces  an  expansion,  a  diminution  of  it  a  contraction.  Light  and  heat, 
therefore,  have  totally  different  offices  to  discharge,  and  the  seats  of  their  respective 
actions  are  in  most  instances  distinct.  In  horizontal  leaves  it  is  chiefly  on  the  upper 
face  that  light  acts ;  this,  looking  towards  the  sky,  is  ready  to  receive  rays  coming  from 
every  part;  it  is  on  the  under  face  that  heat  operates,  directing  in  a  great  measure 
through  the  stomata  the  evaporation  of  water.  An  advantage  for  these  latter  organs 
is,  therefore,  gained.  Exposed  to  the  direct  solar  radiations,  they  would  be  under  con- 
ditions of  perpetual  disturbance,  every  cloud  which  passed  over  the  sun  would  keep 
them  in  a  constant  agitation,  and  interfere  with  their  regularity  of  action.  One  of  the 
most  prominent  differences  between  light  and  heat  is  in  their  mode  of  propagation  through 
bodies;  the  force  of  the  former  is  expended  on  the  point  upon  which  it  falls;  there  is 
nothing  after  the  manner  of  conduction  or  lateral  propagation;  it  is  also  the  same  with 
the  tithonic  rays  (Ap.,  608).    For  this  reason,  when  an  image  falls  on  a  sensitive  plate 


DIFFERENCE  OP  ACTION  ON  THE  UPPER  AND  UNDER  FACE.  IQ3, 

ia  the  camera  obscura,  a  photogenic  copy  is  obtained,  every  hne  being  represented  with 
mathematical  accuracy  and  the  utmost  sharpness;  the  radiation,  having  impinged  upon 
the  plate,  does  not  spread  itself  out  laterally  or  undergo  a  process  of  conduction,  but 
only  produces  its  effect  on  the  portions  which  lie  directly  in  the  lines  of  light.  With 
radiant  heat  it  is  very  different:  a  beam  of  this  agent  falling  upon  a  limited  portion  of 
a  metallic  plate,  slowly  affects  all  the  rest,  for  the  resulting  rise  of  temperature  is  propa- 
gated laterally  from  point  to  point,  and  by-and-by  the  whole  mass  undergoes  an  elevation. 
While  light  and  the  tithonic  rays,  therefore,  are  not  liable  to  conduction,  the  reverse 
holds  in  the  case  of  heat.    Applying  these  reasons  to  the  instance  before  us,  the  rays 
of  light  which  control  the  digestive  operation  could  only  act  in  an  imperfect  way  on 
the  under  side  of  a  horizontal  leaf ;  they  must  produce  their  maximum  effect  on  the 
surface  exposed,  and  operate  elsewhere  through  imperfect  translucency,  finding  their 
way  through  short  distances,  because  the  tissues  are  not  perfectly  opaque.    But  the  rays 
of  heat,  which,  with  the  light,  fall  on  the  upper  face,  are  conducted  with  facility,  or 
transfused  by  warming  the  watery  juices  which  are  circulating  through  the  organ. 
Whatever  changes,  therefore,  take  place  abruptly  in  the  quantity  of  the  incident  rays, 
as  when  clouds  are  suddenly  crossing  the  sun,  are  not  abruptly  fell  upon  the  stomata, 
for  the  leaf,  thin  as  it  is,  acts  as  a  regulator,  and,  from  the  heat  which  it  contains,  sup- 
plies the  momentary  defect;  the  ascending  sap  also,  which  is  coming  through  the  pet- 
iole, is  coming  at  an  elevated  temperature,  for,  in  the  course  of  its  ascent  from  the  roots,  it 
has  been  warmed  in  its  passage  through  the  stem  which  is  enveloped  in  its  dark-colour- 
ed bark — a  colour  well  adapted  to  receive  the  effects  of  the  rays  of  heat.    There  is, 
therefore,  a  reason  why  the  stomata  are  secreted  in  the  shade,  and  why  the  seat  of  the 
digestive  action  lies  on  the  upper  face  of  the  leaf. 

396.  Besides  these  w^ell-marked  differences  of  action  produced  by  the  rays  of  light 
and  of  heat  conjointly  in  the  sunbeam,  there  are  specific  effects  produced  by  the  vari- 
ous coloured  rays;  thus,  the  yellow  seems  to  control  digestion,  and,  as  Dr.  Gardner 
has  proved  (Phil.  Mag.,  Jan.,  1844),  the  blue  ray  motion.  Investigations  of  the  dif- 
ferent effects  produced  by  the  other  rays  of  light,  such  as  the  green,  the  violet,  the  red, 
or  by  those  other  principles  which  are  invisible ;  the  phosphoric  or  the  tithonic  rays 
can  scarcely  yet  be  said  to  have  been  made.  Each  of  these  comprises  rays  differing  in 
constitution,  and  differing  in  refrangibility,  and,  doubtless,  to  each  one  specific  effects 
are  due ;  thus,  it  appears  from  the  experiments  of  Daguerre  and  Neipce,  that  the 
tithonic  rays  have  the  quality  of  changing  the  constitution  of  resinous  bodies.  There 
is,  indeed,  scarcely  one  of  the  productions  of  vegetable  life  which  is  not  characterized 
by  the  facility  with  which  it  undergoes  mutations  under  the  influence  of  these  princi- 
ples; thus,  wax,  which  is  yellow,  bleaches  rapidly;  chlorophyl,  which  is  green,  turns  gray; 
the  volatile  oils  harden,  as  is  often  observed;  the  oil  of  oranges  turns  red;  guiacum,  from 
a  yellowish  brown,  turns  green  or  blue  ;  the  colouring  principle  of  most  petals  is  destroy- 
ed; for  example,  the  beautiful  red  of  carthamus,  and  the  purple  of  violets,  is  rapidly 
bleached ;  woody  fibre  also  undergoes  a  disintegration,  and,  indeed,  there  is  scarcely  a 
vegetable  product  on  which  some  one  of  the  various  radiations  does  not  leave  a  specific 
impression.    The  extensive  series  of  experiments  made  by  Sir  J.  Herschel  on  the 


104 


CENTRALIZATION  OF  NERVOUS  APPARATUS. 


action  of  light  on  different  vegetable  colours  serves  to  prove  that  there  is  scarcely  a  ray 
which  is  not  implicated  in  these  processes;  sometimes  it  is  the  red,  sometimes  the  yel- 
low, sometimes  the  blue.  So,  too,  with  rays  differing  specifically,  the  rays  of  heat  of 
different  refrangibilities,  and  the  tithonic  rays, 

397.  In  the  lower  orders  of  animal  life,  the  controlling  influence  of  the  same  agents 
which  are  thus  so  active  in  the  vegetable  world  is  well  marked;  the  movements  of  the 
polygastric  infusorials  are  well  known  to  be  directed,  to  a  certain  extent,  by  light. 
There  seems  to  be  a  diffused  sensibility  to  that  agent  possessed  by  the  entire  surface 
of  these  beings.  Comparative  anatomists  have  traced  how,  from  this,  which  is  the  ob- 
scurest development  of  specific  nervous  action,  the  nervous  material  begins  to  be  col- 
lected in  centres,  and  locomotive  or  respiratory  gangha  make  their  appearance.  In  the 
case  of  insects,  the  metamorphoses  of  which  are  open  to  our  inspection,  as  they  pass 
successively  through  the  larva  and  pupa  states,  and  finally  reach  their  imago,  or  per- 
fect condition,  so  far  as  the  nervous  system  is  concerned,  all  the  transitions  tend  to  a 
concentration  ;  the  development  of  a  new  instinct,  or  the  production  of  increased  loco- 
motive power,  is  at  once  expressed  by  changes  in  the  magnitude  or  position  of  the  ner- 
vous ganglions,  or  their  connecting  cords.  It  is  true,  that  as  soon  as  this  concentra- 
tion appears,  it  brings  with  it  new  qualities,  and,  in  the  more  elevated  orders  of  life,  one 
of  the  great  functions  of  the  nervous  system  seems  to  be  the  establishing  of  sympathetic 
connexions  between  organs  which  are  carrying  on  processes  that  are  essentially  differ- 
ent ;  between  the  digestive,  the  respiratory,  the  secreting  apparatus.  Among  plants, 
this  high  state  of  centralized  organization  is  not  required ;  the  sap,  driven,  as  we  have 
seen,  by  mere  mechanical  forces,  makes  its  way  from  the  spongioles  to  the  leaves,  and 
then  commences  its  descent.  This  circulatory  movement  is  the  only  bond  of  union 
between  the  various  parts  of  a  vegetable  system.  For  this  reason,  therefore,  botanists 
are  fully  justified  in  their  assertion,  that  a  tree  is  not  a  simple  individual,  but  rather  a 
colony  of  individuals.  It  possesses  no  interior  nervous  system,  the  oftice  of  which  is 
to  bring  into  connexion  parts  that  are  distant,  because  the  type  upon  which  it  is  con- 
structed requires  no  such  machinery. 

398.  But  there  are  considerations  which  enable  us  to  trace  analogies  between  the 
mode  of  action  of  the  nervous  principle,  even  in  the  most  elaborate  forms,  with  those 
more  obscure,  which  belong  to  the  vegetable  world.  As  has  been  fully  proved  by  phys- 
iologists, all  the  nerves  of  special  sensation  take  their  origin  amid  the  minute  ramifica- 
tions of  bloodvessels ;  and  it  is  the  changes  which  occur  in  the  circulating  fluid  which 
impress  a  specific  action  on  the  nervous  terminations — an  action  at  once  transmitted  to 
the  centres,  and  there  disposed  of.  Thus,  by  the  heart,  venous  blood,  from  which  the 
elements  of  carbonic  acid  have  to  be  expelled,  is  thrown  into  the  minute  ramification 
of  the  pulmonary  artery,  and  there,  coming  in  contact  with  the  terminal  fibres  of  the 
jJar  vagi/m,  it  impresses  upon  them  an  influence — an  influence  which  is  in  an  instant 
propagated  to  the  respiratory  nervous  centres,  and  there  is  expended  in  producing  a 
reflex  effect.  Through  the  proper  nervous  channels,  the  diaphragm,  the  intercostals, 
and  the  various  muscles  engaged  in  respiration,  are  put  in  action  ;  the  capacity  of  the 
chest  increases ;  a  few  cubic  inches  of  atmospheric  air  enter  the  trachea,  and  fill  the 


/ 


ANALOGIES  BETWEEN  NERVOUS  AND  IMPONDERABLE  ACTION.  195 

larger  bronchial  tubes.  lu  an  instant,  by  diftusion  (Ap.,  50),  the  oxygen  of  this  air  leaves 
its  nitrogen,  which  in  these  circumstances  is  a  more  slowly-diffusing  gas,  finds  its  way 
to  the  minutest  air-cells,  passes  in  an  instant  (Ap.,  53)  through  the  thin  vascular  coats, 
interchanges  with  carbonic  acid,  and  the  act  of  arterialization  is  complete.  The  arte- 
rialization  of  the  blood  is,  therefore,  nothing  more  than  an  ordinary  chemical  process, 
and  the  nervous  agency  which  has  been  brought  into  action  is  for  no  other  end  than 
the  production  of  a  mechanical  effect ;  for,  though  the  act  of  respiration  is  eventually 
carried  on  under  the  operation  of  chemical  laws,  mechanical  movements  have  to  pre- 
cede. A  division  of  those  nerves  puts  an  end  to  the  process,  not  because  respiration 
is  the  result  of  a  vital,  a  nervous,  or  any  other  such  agency,  but  simply  because  the 
necessary  movements  which  end  in  the  introduction  of  oxygen  gas  can  no  longer  be 
accomplished. 

399.  Now  let  us  turn  to  the  vegetable  kingdom.  As  long  as  the  sun  is  above  the 
horizon,  his  beams,  impinging  upon  everything,  occasion  an  elevation  of  temperature. 
The  various  objects  exposed — the  leaves  of  trees,  the  grass,  and  the  surface  of  the 
ground — become  warm.  Participating  in  this  elevation  of  temperature,  the  atmo- 
spheric air  has  its  capacity  for  vapour  increased,  and  that  transpiration  of  steam  which 
is  copiously  going  forward  from  the  exposed  vegetable  surfaces,  from  the  soil,  and 
from  water,  raises  the  dew  point.  But  on  the  going  down  of  the  sun,  and  even  before 
the  close  of  day,  a  reverse  action  begins  to  be  established.  Radiation  into  space  is  ac- 
complished, the  temperature  of  the  ground  and  of  the  leaves  begins  to  fall,  and  by-and- 
by  the  dew  point  is  reached,  and  now  drops  of  water  form  on  those  organs,  and  under 
its  influence  they  begin  to  recover  the  parching  effect  of  the  preceding  day.  The  re- 
duction of  the  atmospheric  temperature  down  towards  the  dew  point,  thus  brought 
about  by  the  conjoint  agency  of  the  ground  and  the  exposed  parts  of  plants,  in  an  in- 
stant puts  a  check  upon  evaporation  from  the  leaves ;  this,  in  its  turn,  reacts  upon  the 
ascent  of  the  sap ;  it  is  like  putting  the  cover  over  the  burner  of  a  spirit  lamp ;  evapo- 
ration from  the  reservoir  is  stopped,  and  simultaneously,  also,  the  current  along  the  cap- 
illary fibres  of  cotton  is  checked.  With  this  check  upon  the  ascent  of  the  sap,  that 
store  of  carbonic  acid  gas  which  exists  in  the  soil,  and  which  arises  from  the  decay  of 
humus,  or  from  artificial  manures,  is  also  preserved ;  carbonic  acid  which,  during  the 
day,  comes  up  to  the  leaves  with  the  circulating  juice,  and  is  decomposed  by  solar  light. 
But,  now,  if  a  cloud  intervenes,  or  the  sky  at  night  be  uniformly  obscured,  no  radiation 
can  be  accomplished,  uo  reduction  of  temperature  takes  place,  the  dew  point  is  never 
reached,  and  evaporation  goes  on  from  the  leaves,  and  carbonic  acid  gas,  drawn  up 
from  the  spongioles,  is  expended.  The  reflex  action  of  that  radiant  heat  which  came 
from  the  sun  during  the  day  is  cut  off,  its  passage  outward  into  the  regions  of  space 
arrested,  and  the  dependant  physiological  phenomena  cease  to  be  performed.  There 
is  an  analogy  of  effect  between  the  temporary  action  of  an  overhanging  canopy  of 
clouds  at  night  on  the  vegetable  world,  and  the  permanent  injury  which  results  to  an 
animal  when  thenar  vagum,  one  of  the  offices  of  which  is,  at  proper  moments,  to  in- 
troduce a  measured  quantity  of  oxygen  gas  into  the  system,  is  divided. 

400.  Where,  then,  do  these  things  carry  our  reflections  I    What  are  the  elevated 

O 


106 


PLANTS  ARE  THE  RESULTANT  OF  IMPONDERABLE  ACTION. 


ideas  they  bring  before  us  1  Do  they  not  show  that  the  great  spaces  of  the  universe 
are  not  empty  soUtudes,  in  which  there  only  reside  mechanical  forces,  in  which  only 
the  influences  of  gravitation  and  projectile  action  occur  1  Do  they  not  teach  us,  that 
wherever  a  ray  of  light  can  pass,  there  is  the  capability  for  organization  and  life?  And 
of  those  innumerable  stars  which  we  see  at  night,  some  of  which  are  giving  forth  rays 
of  one,  and  some  of  another  colour,  and  a  multitude  of  double  stars,  which  furnish  com- 
plementary lights  to  their  attendant  planets,  who  can  tell  what  multiplied  results  these 
things  impress  on  the  world  of  organization  1  In  our  reflections  on  the  constitution 
of  the  universe,  though  the  beautiful  perfection  of  its  mechanism  may  excite  our  won- 
der, do  not  these  views  of  its  capability  for  organization,  of  the  constant  presence  of 
light,  the  parent  of  life,  call  for  our  unbounded  admiration  1  Instead  of  regarding  the 
interplanetary  spaces  as  a  great  vacuum,  a  desolate  solitude,  they  rise  before  us  as  re- 
gions filled  with  active  forces,  and  ready  to  put  on  or  to  communicate  movement  and  life. 

401.  Mental  volitions  are  executed  by  muscular  movements  arising  from  the  passage 
of  some  agent  along  the  nerves,  an  agent  which  does  not  require  any  perceptible  time 
for  its  transit,  but,  within  the  spaces  which  we  have  under  consideration,  seems  to  act 
instantaneously — a  volition  originates,  and  contemporaneously  a  motion  is  accomplish- 
ed. The  speed  with  which  the  different  radiani  principles  are  propagated  through  the 
ether,  rivals  the  speed  of  nervous  movement ;  spaces  such  as  those  which  we  ordinarily 
have  to  do  with  are  passed  over  in  an  inconceivably  short  period  of  time. 

402.  It  is  not  without  abundant  reason  that  we  are  thus  led  to  describe  the  solar 
radiation  as  discharging  the  part  of  a  vegetable  nervous  agent,  which  simulates,  in  many 
of  its  operations,  the  nervous  principle  of  animals.  Out  of  a  limited  number  of  ponder- 
able substances,  such  as  carbon,  hydrogen,  nitrogen,  oxygen,  and  a  few  others,  aU  kinds 
of  organized  structures  are  formed,  but  then  there  is  an  extensive  machinery  to  collate 
and  group  together  these  different  bodies.  Light,  in  itself,  can  produce  as  many  different 
effects  as  there  are  possible  combinations  of  colour,  for  each  one  of  its  rays  has  pecu- 
liar powers  of  its  own,  and  it  is  also  attended  by  other  invisible  and  imponderable  prin- 
ciples which  have  their  modes  of  action.  An  organized  structure  of  a  given  kind  is, 
therefore,  the  result  of  the  operation  of  many  of  these  forces,  and  is  an  expression  of 
their  aggregate  action.  In  the  full  development  of  a  perfect  tree  there  has  been  ex- 
pended a  measured  quantity  of  forces,  of  light,  or  of  heat,  and  the  organized  mass,  as  it 
stands  before  us,  the  product  of  those  forces,  is  the  resultant  of  millions  of  vibrations  of 
the  luminiferous  ether  which  have  acted  upon  ponderable  atoms  ;  vibrations  which  have 
stood  in  a  certain  relation  to  each  other,  as  the  symmetry  of  the  vegetable  parts  indi- 
cates. In  the  operations  of  human  agency,  something  of  the  same,  though  of  a  grosser 
kind,  may  be  seen.  We  have  not,  it  is  true,  the  power  of  calhng  into  existence,  or  of 
determining  in  an  enduring  shape,  or  of  giving  an  imbodied  form  to  material  atoms; 
but  in  the  same  manner  that  Nature,  operating  through  ethereal  undulations,  creates  the 
various  forms  of  vegetable  life,  there  has  been  committed  to  us  a  similar  control  over 
those  grosser  undulations  which  move  in  the  atmospheric  air.  The  imagination,  the 
genius  of  the  great  masters  of  music,  have  already  grouped  together  combinations  of 
these  waves,  which  are  destined  to  an  earthly  immortality  ;  combinations  which,  when 


PLANTS  ARE  THE  RESULTANTS  OF  IMPONDERABLE  ACTION.  IQJ 

once  heard,  leave  their  indelible  impression  on  the  memory,  and  are  to  us  an  imbodi- 
ment  of  symmetry  and  harmony.  These  ideal  creations,  which  exist  only  for  the  mind, 
are  analogous,  in  very  many  points  of  view,  to  those  more  tangible  creations  which  are 
formed  by  ethereal  waves,  and  which  Nature  has  reserved  in  her  own  hands.  The 
symmetrical  or  beautiful  forms  which  are  transmitted  to  the  brain  by  the  eye,  appeal  at 
last  to  that  same,  that  common  principle,  which  receives  melodious  or  harmonious 
sounds  transmitted  by  the  ear  ;  and  the  creations  of  human  genius,  whether  they  be  ex- 
pressed in  the  language  of  music  or  of  painting,  whether  they  are  heard  in  the  cathe- 
dral, or  seen  in  the  canvass  of  Claude  Lorraine,  give  us  pleasure,  because  their  final 
impression  is  made  on  a  mathematical  organ  which  is  so  constructed  as  to  appreciate 
whatever  is  symmetrical  in  position,  whatever  is  graceful  in  figure,  whatever  is  harmo- 
nious in  movement. 

403.  From  this  point  of  view,  therefore,  I  look  upon  the  vegetable  world  as  an  im- 
bodiment  of  the  action  of  ethereal  agents.  A  tree,  when  covered  with  blossoms  in 
spring,  or  loaded  with  fruit  in  autumn,  is  a  resultant  of  the  play  of  those  active  forces 
which  have  been  emitted  by  the  sun,  an  expression  of  what  has  been  done  by  vibratory 
movements  operating  on  ponderable  molecules.  As  soon  as  the  young  plant  has  put 
its  ascending  axis  above  the  ground  and  exposed  itself  to  the  solar  beam,  growth  rap- 
idly begins  to  take  place,  and  organized  matter  to  be  condensed  from  the  air ;  and  now 
a  green  colour  is  developed,  and  the  stem  elongates,  and  leaves  are  put  forth.  At  the 
proper  epoch  the  reproductive  organs  are  evolved  and  flowers  appear,  and  by  the  end 
of  its  annual  period  the  plant  has  laid  up  a  store  of  nutritive  matter  for  the  ensuing 
spring,  or,  if  such  be  its  habit,  has  provided  for  the  germs  it  has  called  into  existence. 
In  carrying  forward  all  those  multiplied  operations  which  have  ended  in  these  events, 
its  leaves  and  its  stem  have  gone  upward  in  search  of  the  light — light  which  has  sym- 
metrically arranged  their  parts  and  furnished  their  substance.  But  these  general  views 
are  far  from  giving  us  an  accurate  idea  of  the  forces  which  have  been  expended,  or 
the  motions  which  have  been  executed  in  producing  the  result  we  contemplate.  An 
exogenous  forest  tree,  from  its  magnitude,  rising,  perhaps,  a  hundred  feet  above  the 
ground,  and  spreading  its  branches  over  hundreds  of  square  yards,  may  impress  us  with  a 
sense  of  sublimity  ;  a  section  of  its  stem  might  assure  us  that  it  had  lived  for  a  thousand 
years,  and  its  total  weight  could  only  be  expressed  by  tons.  An  object  like  this  may, 
indeed,  call  forth  our  admiration,  but  that  admiration  is  expanded  into  astonishment 
Avhen  we  come  to  consider  niinutelv  the  circumstances  which  have  been  involved  in 
producing  the  result.  If  we  conceive  a  single  second  of  time — the  beat  of  a  pendu- 
lum— divided  off  into  a  million  of  equal  parts,  and  each  one  of  these  inconceivably 
brief  periods  divided  again  into  a  million  of  other  equal  parts,  a  wave  of  yellow  light 
during  one  of  ihese  last  small  intervals  has  vibrated  five  hundred  and  thirty-five  times. 
And  now  that  yellow  light  is  the  agent  which  has  been  mainly  involved  in  building  up 
the  parts  of  the  tree,  in  fabricating  its  various  structures,  and  during  every  one  of  a 
thousand  summers,  from  sunrise  to  sunset,  the  busy  rays  have  been  carrying  on  their 
operation — who,  then,  can  conceive,  when,  in  the  billionth  of  a  second,  such  enormous 
numbers  of  movements  are  accomplished,  how  many  have  been  spent  in  erecting  an 


108 


CONCLUSION. 


aged  forest  oak !  Who  also  can  conceive  the  total  amount  of  force  employed  from 
century  to  century  in  arranging  the  vegetation  of  the  surface  of  the  globe  ! 

404.  I  therefore  regard  a  planetary  body  like  the  earth,  in  its  orbitual  revolution 
round  the  sun,  as  a  predetermined  focal  centre  on  which  the  emanations  of  that  star 
shall  be  expended;  first,  in  producing  vegetable  organization,  and,  finally,  in  lending  their 
aid  to  the  evolution  of  animal  intellect.  The  forces  which  Newton  revealed  as  urging 
such  a  body  forward,  or  causing  it  to  glide  in  its  elliptic  path,  appear  only  as  an  incidental, 
though  essential  part  of  the  mechanism  of  the  universe,  the  interest  of  which  disap- 
pears in  that  higher  interest  which  must  attach  to  whatever  stands  in  intimate  connex- 
ion with  organization  and  vitality.  Those  many-coloured  luminous  wavelets  which 
are  ceaselessly  crossing  the  interplanetary  spaces,  go  forward  on  an  appointed  errand, 
and  sooner  or  later  discharge  their  final  task;  nor  are  the  planets  in  the  solar  system  a 
colony  of  opaque  globes,  rotating,  without  purpose  or  end,  around  the  central  attractive 
mass.  The  solar  system  is  an  orb  of  movement  and  light,  full  of  vibrations  of  every 
tint  visible  and  invisible,  and  which  here  and  there  envelops  and  enshrouds  revolving 
points  of  organization  and  life. 


GENERAL  APPENDIX, 

CONTAINING  THE 

EXPERIMENTAL  DOCUMENTS. 


EXPERIMENTS,  &c. 


CHAPTER  1. 

EXPERIMENTS  MADE   TO  DETERMINE  WHETHER  LIGHT  EXHIBITS  ANY  MAGNETIC  ACTION. 

{From  the  Journal  of  the  Franklin  Institute  for  February,  1835.) 

Contents  :  Character  of  the  Sky  in  Virginia. — Exa?nination  of  Mr.  Christie's  Ex- 
periment.— Needles  not  affected  by  the  Violet  Rays. — No  Reaction  between  a  Magnet 
or  Voltaic  Currents  and  Light. 

1.  "  The  more  refrangible  rays  of  light  are  said  to  possess  the  property  of  rendering 
iron  and  steel  magnetic.  The  existence  of  this  property  was  first  asserted  by  Dr.  Mo- 
richini,  of  Rome.  Other  observers  subsequently  failed  in  obtaining  the  same  results ; 
but  in  the  year  1825,  the  fact  appeared  to  be  decisively  established  by  the  learned  and 
accomplished  Mrs.  Somerville,  in  an  essay  published  in  the  Transactions  of  the  Royal 
Society.  In  her  experiments,  sewing  needles  were  rendered  magnetic  by  exposure  for 
two  hours  to  the  violet  ray,  and  the  magnetic  virtue  was  communicated  in  still  shorter 
time  when  the  violet  rays  were  concentrated  by  means  of  a  lens.  The  indigo  rays  were 
found  to  possess  a  magnetizing  power  almost  to  the  same  extent  as  the  violet,  and  it 
was  observed,  though  in  a  less  degree,  in  the  blue  and  green  rays.  It  is  wanting  in  the 
yellow,  orange,  and  red.  Needles  were  likewise  rendered  magnetic  by  the  sun's  rays 
transmitted  through  green  and  blue  glass.  These  results  have  been  verified  by  M.  Zan- 
tedeschi,  of  Pavia  {Bihl.  Univ.  for  May,  1829),  but  their  accuracy  has  been  doubted  by 
Messrs.  Riess  and  Moser,  who  consider  that  the  means  employed  by  Mrs.  Somerville 
for  ascertaining  the  magnetic  state  of  the  needles  were  not  sufficiently  exact.  They 
found  the  oscillation  of  needles  to  be  wholly  unaffected  by  exposure  to  the  prismatic 
colours.  (Brewster  s  Journal,  ii.,  p.  225,  N.  S.)  This  must  still  be  regarded,  therefore, 
as  one  of  the  disputed  points  in  science." — (^Turner.) 

2.  It  has  been  supposed  that  this  disparity  of  results  arose  entirely  from  local  circum- 
stances. A  hazy  atmosphere,  such  as  is  met  with  in  the  northern  and  middle  countries 
of  Europe,  might  perhaps  influence,  in  some  manner,  this  peculiar  property  of  light,  when 
the  clearer  sky  of  Italy  allowed  an  opportunity  of  making  the  experiment.  Some,  in- 
deed, have  thought  that  the  observers,  who  were  said  to  have  verified  the  original  re- 
sults of  the  Italian  philosopher,  were  themselves  deceived  in  not  having  previously  as- 
certained the  magnetic  state  of  the  needles  they  used.  That  mistakes  of  this  and  a 
similar  kind  are  easily  made,  will  appear  in  the  course  of  this  communication. 

3.  During  the  past  summer,  I  have  attempted  to  satisfy  myself  whether  the  more  re- 
frangible rays  really  exert  any  magnetic  influence ;  and  happening  to  reside  in  the  south 


4 


CHARACTER  OF  THE  SKY  IN  VIRGINIA. 


of  Virginia,  upon  the  same  parallel  of  latitude  as  Tunis  and  the  more  northerly  African 
kingdoms,  I  thought  the  situation  too  favourable  to  suffer  such  an  opportunity  to  pass 
without  endeavouring  to  gain  some  decisive  information  on  this  contested  point. 

4.  The  sky  of  that  part  of  Virginia  is  not,  however,  so  bright  as  might  be  expected. 
WJien  unclouded,  it  is  of  a  clearer  blue  than  the  sky  of  England,  and,  I  think,  rather 
darker,  approaching,  by  several  shades,  nearer  to  the  tint  of  Prussian  blue  than  that 
azure  colour  which  it  has  in  the  latter  country.  During  a  residence  of  twelve  months, 
1  saw  it  twice  of  that  intense  complexion  which  they  say  it  has  in  the  tropical  regions. 
The  moon  appeared  globose,  and  her  centre  to  bulge  out  like  a  ball,  and  the  planet 
Venus  might  be  fancied  to  exhibit  a  well-formed  crescent.  The  air  passes  from  great 
moisture  to  dryness  with  rapidity  ;  a  good  barometer  is  seldom  at  rest.  The  tempera- 
ture of  the  sunbeam  is  sometimes  132  F.,  but  the  siphon  barometer  seldom  varies  more 
than  one  inch.  Clouds  form  and  rise  with  great  rapidity,  the  seasons  are  uncertain, 
and  the  atmosphere  is  often  deformed  with  a  mistiness  that  obscures  distinct  vision. 
But  the  dense  clouds  that  gather  in  the  west  of  an  evening  emulate  the  beauty  of  a 
sunset  on  the  Atlantic  Ocean. 

5.  I  am  thus  particular  in  describing  the  state  of  the  atmosphere  in  which  these  ex- 
periments were  made,  because  much  stress  has  been  laid  upon  that  circumstance,  as  ma- 
terially affecting  the  results  of  magnetic  action  of  the  solar  ray.  And,  as  the  experiments 
now  to  be  detailed  lead  to  a  negative  decision,  it  is  well  to  explain  under  what  circum- 
stances they  were  performed,  that  those  who  have  come  to  an  opposite  conclusion  may 
have  an  opportunity  of  pointing  out  whether  it  is  really  owing  to  the  state  of  the  atmo- 
sphere. 

6.  In  the  year  1824,  Mr.  Christie  found  that  a  needle  six  inches  long,  contained  in 
a  brass  compass  box  with  a  glass  cover,  suspended  by  a  hair,  and  made  to  vibrate,  al- 
ternately shaded  and  exposed  to  the  sun,  came  to  rest  much  sooner  in  the  latter  than 
in  the  former  case.  That  this  was  not  occasioned  by  an  increase  of  temperature,  was 
proved  by  the  needle  vibrating  more  rapidly  when  its  temperature  was  raised  by  other 
means. 

7.  In  repeating  this  experiment,  I  very  quickly  found  that  it  depended,  in  a  great 
measure,  on  the  nature  of  the  suspension  of  the  needle,  and  its  position  with  respect  to 
the  incident  light,  what  results  would  be  obtained.  If  the  needle  was  suspended  on  a 
point  or  by  a  thread,  without  torsion,  both  the  time  and  the  number  of  vibrations  were 
the  same,  whether  the  needle  was  exposed  to  the  sunbeam  or  not.  But,  if  the  needle 
was  suspended  by  a  hair  or  other  organic  substance,  having  torsion,  the  sunbeam  would 
occasion  a  degree  of  twist  in  the  hair  on  its  first  exposure  to  hght ;  and  if  the  direc- 
tion of  that  twist  happened  to  coincide  with  the  direction  of  the  needle's  motion,  of 
course  the  momentum  of  the  needle  was  increased,  and  the  vibrations  continued  longer. 
A  needle  which  vibrated  forty-four  times  in  one  minute,  would  occasionally,  owing  to 
this  cause,  vibrate  nearly  forty-six  when  suspended  by  a  hair  ;  but  if  by  a  silk  fibre,  its 
vibrations  were  always  forty-four,  the  first  arc  of  vibration  being  in  every  instance  40". 
That  this  action  was  due  to  a  twist  of  the  hair,  as  a  hygrometric  body,  I  ascertained 
by  means  of  a  simple  arrangement.    To  a  brass  cross-piece,  A  {fig.  l,pl.  1),  supported 


EXAMINATION  OF  MR.  CHRISTIE'S  EXPERIMENT.  5 

on  a  stand  by  two  pillars,  B  C,  a  hair,  e,  was  fastened,  about  the  point  A ;  its  other 
extremity,  g-,  being  fastened  to  a  whalebone  spring,  D,  by  a  thread,/;  the  extremity,  g, 
of  the  hair,  being  bent  by  the  knot  on  one  side,  served  as  an  index.  On  exposing  the 
instrument  to  the  sunbeam,  the  little  index,  g,  immediately  moved,  sometimes  more  than 
a  semicircle. 

8.  Thinking  to  obtain  more  decisive  effects,  I  concentrated  the  sunbeam  with  a  lens 
on  the  south  pole  of  the  suspetTled  needle,  and  found  that  the  needle  was  thrown  into 
a  rapid,  tremulous  motion.  But  here  the  hot  air  ascending  from  the  needle,  acts  upon 
it  as  upon  the  sail  of  a  windmill ;  and  the  same  effect  ought  to  take  place,  to  a  certain 
extent,  on  simple  exposure  of  the  half  of  a  vibrating  needle  to  direct  light.  But  I  found 
that  a  needle  suspended  in  the  vacuum  of  an  air-pump  by  a  thread  without  torsion,  is 
in  no  way  affected  by  exposure  to  solar  light. 


Ei-. 

Length  of  needle  in 
inches. 

Nu.  of  vibrations  in  shade. 

1.  the  sunbeam. 

1 

H  sewing  needle 

44 

44 

2 

2  blue  spring 

32 

32 

3 

31  do. 

26 

26 

Therm,  in  the  sun,  109  to  105.— Barom.,  29.3.— Duration  of  Exp  ,  60" 


9.  It  is  said  expressly  in  the  account  of  Christie's  experiment,  that  the  needle  was 
contained  in  a  brass  compass  box.  It  might  have  been  that  electrical  currents  were 
excited  in  that  box,  which  was  the  cause  of  the  derangement  in  question ;  I  therefore 
vibrated  a  needle,  under  similar  circumstances,  with  the  same  result  as  above  stated.  I 
should  mention  that  this  was  done  in  a  solid  cylinder  or  ring  of  brass,  without  any  seam 
or  soldered  junction ;  but  as  compass  boxes  are  generally  made  of  sheet  brass,  with  a 
soldered  seam  in  the  side,  it  was  barely  possible  that  the  fine  line  of  solder  acted  with  the 
brass,  as  a  thermo-electric  couple,  capable  of  excitation  by  the  warmth  of  the  sunbeam. 
I  therefore  made  a  compound  cyhnder  of  copper  and  zinc,  c  z  {fig.  2,  jV.  ]),  the  edges 
of  which,  at  a  and  Z>,  were  neatly  soldered,  the  junction  h  being  highly  polished,  and  that 
at  a  blackened.  The  needle  was  suspended  in  an  exhausted  receiver,  by  a  silk  fibre,  g, 
and  a  ray  of  light,  c  d,  coming  from  an  aperture  half  an  inch  wide  in  the  shutter,  fell 
upon  the  junction.  The  needle  used  in  this  experiment  was  of  watch-spring ;  its  first 
vibration  was  performed  in  an  arc  of  forty  degrees,  and  when  the  compound  cylinder  was 
taken  away,  it  made  thirty-two  vibrations  in  sixty  seconds  in  vacuo.  On  placing  it 
concentrically  with  the  compound  cylinder,  and  suffering  the  ray  to  impinge  on  the 
polished  junction  the  moment  that  the  arc  of  vibration  had  become  forty  degrees,  the 
number  of  oscillations  in  one  minute  was  carefully  observed;  six  experiments  gave  sev- 
erally the  number  thirty-two.  On  turning  the  blackened  junction  to  the  light,  the  re- 
sult was  still  thirty-two ;  and  on  substituting  the  solid  brass  cylinder,  three  consecutive 
trials  gave  thirty-two.  The  thermometer  stood  in  the  sunbeam  at  103  ;  the  barometer 
at  28.8. 

]  0.  By  some  this  magnetic  action  of  light  has  been  attributed  to  the  violet  or  more 
refrangible  rays  only.  A  needle  made  of  watch  spring,  about  four  inches  long,  which, 
in  an  exhausted  receiver,  suspended  by  a  filament  of  silk,  exhil)ited  no  polarity,  had  one 
half  of  it  exposed  to  the  violet  ray,  cast  by  an  equi-angular  prism  of  flint  glass.    This  ray 


NO  REACTION  BETWEEN  A  MAGNET  OR  VOLTAIC  CURRENTS  AND  LIGHT. 


was  separated  from  the  others  by  passing  through  a  sUt  in  a  metallic  screen,  and  half 
the  needle  shielded  from  its  action  by  a  piece  of  paper.  After  two  hours'  exposure  to 
the  sun,  it  was  suspended  again  in  the  exhausted  receiver,  but  still  showed  no  token  of 
polarity  ;  it  was  then  exposed  to  the  other  rays  successively,  with  the  same  result.  The 
needle  was  now  slightly  touched,  and,  slowly  vibrating,  arranged  itself  along  the  mag- 
netic meridian.  The  first  vibration  was  performed  in  a  semicircular  arc,  and  the  num- 
ber of  vibrations  performed  during  one  hundred  oscillations  of  a  seconds'  pendulum  was 
twenty-seven.  But,  after  four  hours'  exposure  to  the  violet  ray,  as  before,  no  evidence 
of  any  change,  either  increasing  or  diminishing  the  number  of  oscillations,  could  be 
gained.  A  column  of  violet  light,  passing  through  a  disk  of  stained  glass,  was  con- 
centrated on  one  end  of  a  sewing  needle  by  means  of  a  lens,  without  producing  any 
change  in  the  number  of  vibrations  it  made  in  one  minute.  This  needle,  on  some  oc- 
casions, however,  would  give  -unequal  results ;  when  its  first  vibration  was  performed 
in  a  semicircle,  the  number  varied  from  forty-one  to  forty-three  in  sixty  seconds.  On 
vibrating  it  in  vacuo,  its  results  uniformly  gave  the  latter  number  very  nearly. 

11.  The  position  of  the  needle  to  the  incident  ray  is  not  of  any  consequence,  whether 
it  receives  it  obliquely  in  the  direction  of  the  light  or  across  it.  If  soft  iron  be  substi- 
tuted for  steel,  the  results  are  still  negative,  even  if  the  needle  be  arranged  in  the  mag- 
netic meridian,  the  line  of  dip,  or  any  other  position.  I  therefore  come  to  the  conclu- 
sion that  the  violet  ray,  as  developed  by  a  prism  of  English  flint  glass,  possesses  no  in- 
fluence on  the  magnetic  needle,  and  all  the  other  rays  are  equally  inert. 

12.  But,  as  Mrs.  Somerville  found  that  a  needle  placed  under  a  piece  of  glass  or  blue 
riband,  having  half  its  length  protected  by  paper,  became,  in  a  short  time,  magnetic,  I 
tried  the  same  experiment,  but,  in  every  instance,  failed  in  making  the  needle  magnetic. 
When  suspended  by  a  silk  fibre  in  vacuo,  needles  showed  no  disposition  to  arrange 
themselves  in  any  particular  line,  and  when  they  came  to  rest,  they  were  found  cutting 
the  magnetic  meridian  at  every  angle,  although  the  temperature  of  the  sunbeam  to  which 
they  had  been  exposed  on  one  occasion  was  124°  F.  Great  care  was  taken  to  ascer- 
tain the  previous  non-magnetic  state  of  the  needles,  and  they  were  suspended  by  a  fibre 
without  torsion.  To  ascertain  whether  anything  was  due  to  the  nature  of  the  medium, 
I  substituted  prisms  of  water,  alcohol,  spirit  of  turpentine,  and  other  essential  oils,  with 
the  same  results. 

13.  These  are  the  means  by  which,  it  is  said,  the  magnetism  of  light  was  first  dis- 
covered ;  but  there  are  much  more  dehcate  methods  of  detecting  such  an  action  if  it 
existed,  and  to  them,  in  the  next  place,  I  resorted.  For,  if  the  violet  or  other  rays  of 
light  exercise  an  influence  on  the  magnetic  needle,  that  action  must  be  mutual  between 
them,  and  the  light,  in  its  turn,  should  suffer  a  derangement.  To  ascertain  this,  I  ad- 
mitted a  divergent  beam  of  light  through  a  hole  in  the  shutter  of  a  dark  room  ;  the  cone 
of  luminous  matter  at  its  apex  was  about  yV^li  i^^ch  in  diameter,  and  a  hair  or  oth- 
er filament  held  in  it  exhibited  the  phenomena  of  diffi-action,  the  colours  being  received 
into  the  eye  by  a  lens.  Across  this  beam  a  silver  wire  was  adjusted,  each  of  its  ex- 
tremities connected  with  cups  of  mercury,  which  communicated  with  the  poles  of  a 
voltaic  battery.    It  was  expected  that,  if  there  was  any  action  between  a  magnetic  fila- 


NO  REACTION  BETWEEN  A  MAGNET  OR  VOLTAIC  CURRENTS  AND  LIGHT.  7 

ment  aud  light,  some  derangement  would  be  seen  in  the  diffracted  fringes  when  the 
current  passed  ;  but  none  such  was  observable. 

14.  Again,  two  wires  were  so  adjusted  that  they  could  be  made  to  approach  or  re- 
cede from  each  other  by  a  screw  movement,  aud  voltaic  currents  be  passed  in  either 
direction,  up  or  down  them,  conjointly  or  separately  ;  when  they  were  within  the  rloth 
part  of  an  inch,  the  fringes  they  produced  were  very  perceptible,  but  the  passage  of  the 
current  caused  no  alteration  whatever.  These  experiments  were  also  repeated  in  ho- 
mogeneous light,  with  the  same  results. 

15.  A  hollow  prism,  a  h  {Jig.  3,  pi.  1),  for  containing  transparent  but  in)perfectly 
conducting  liquids,  was  traversed  by  a  voltaic  current  from  the  battery,  c  z,  by  means  of 
platina  wires,  e  e,  one  extremity  of  each  of  which  went  to  the  bottom  of  the  liquid,  and 
the  other  dipped  into  the  mercurial  cups  of  communication,  d  d.  A  ray  of  light, 
passed  through  the  prism,  and  was  refracted  on  the  screen,  g  I,  or  viewed  by  a  glass. 
Some  of  those  lines  which  traverse  the  spectrum  were  at  times  visible  ;  but  neither 
these  nor  the  spectrum  itself  suffered  any  perceptible  alteration  during  the  passage  of 
the  current. 

16.  Some  philosophers  have  pointed  out  the  different  degrees  of  temperature  which 
Sir  W.  Herschel  detected  in  different  parts  of  the  prismatic  spectrum,  as  analogous  to 
that  increase  of  temperature  which  takes  place  in  the  cells  of  the  voltaic  battery,  as  we 
proceed  from  the  negative  to  the  positive  pole.  Hence,  they  have  supposed  that  each 
end  of  the  spectrum  was  in  an  opposite  electrical  state.  With  the  analogy  I  have 
nothing  to  do  ;  but  to  ascertain  whether  the  deduction  from  it  is  founded  in  fact,  I 
made  this  experiment.  I  received  into  a  tube,  a  h  {fig.  4,  pi.  1),  filled  with  acidu- 
lated water,  the  whole  of  the  prismatic  spectrum  cast  by  an  eq-uiangular  prism  of  Eng- 
lish flint  glass,  in  that  part,  c,  which  was  covered  by  the  violet  ray ;  I  immersed  one 
wire,  e  e  e,  oi  a  galvanometer,  arranging  the  other  wire,/  f,  in  that  part,  d,  which  re- 
ceived the  red  ray.  Alternately  placing  and  withdrawing  the  arrangement  from  this 
position,  it  was  to  be  expected  that  either  a  continuous  current  or  a  wave  of  electricity 
would  pass  along  the  galvanometer  wires,  producing  either  a  permanent  or  transient  dis- 
turbance. I  was  not  able,  however,  to  notice  such  an  effect ;  and  suspecting  that,  if 
any  electricity  was  developed,  it  might  be  of  such  low  intensity  as  to  be  unable  to  pass 
through  acidulated  water,  I  repeated  the  experiment,  using,  instead  of  the  tube  of  wa- 
ter, a  piece  of  polished  tin,  in  the  shape  of  a  parallelogram,  three  inches  long  and  one 
broad,  to  each  end  of  which  the  galvanometer  wires  were  soldered  ;  but  the  results  were 
still  negative. 

17.  The  general  termination  of  these  experiments  would  lead  us  to  suppose,  that  if 
there  be  any  reaction  between  galvanic  or  magnetic  currents  and  solar  light,  they  are  of 
such  a  nature  as  to  forbid  repetition  in  latitudes  as  far  south  as  35". 


DESCRIPTION  OF  THE  PHENOMENON. 


CHAPTER  11. 

OF  THE   TIDAL  MOTIONS   OF  MOVABLE  ELECTRIC  CONDUCTORS. 

(From  the  Journal  of  the  Franklin  Institute  for  January,  1836.) 

Contents  :  Desci'iption  of  the  Phenomenon. —  The  Polar  Wires  act  as  Centres  of  At- 
traction.—  They  'produce  Tides. — Cause  of  the  Oscillations. —  Cause  of  the  Spiral 
Motions. 

18.  "Dans  d'autres  circonstances  on  observe  encore  au  milieu  des  masses  liquides, 
des  mouvemens  singuliers  qu'il  est  excessivement  difficile  de  decrire,  tant  ils  sont  noin- 
breux  et  changeans.  Je  vais  essayer  d'en  donner  une  idee,  en  remarquant  toutefois, 
qu'apres  avoir  fait  de  nombreuses  experiences  sur  ce  sujet,  il  m'a  ete  impossible  d'en 
saisir  la  loi." — {Pouillet?) 

19.  The  singular  movements  here  spoken  of  by  Pouillet  have  likewise  drawn  the 
attention  of  several  other  philosophers.  Erman  and  SeruUas  have  both  recorded  in- 
stances of  gyratory  motion  produced  in  certain  bodies,  especially  mercury,  by  the  con- 
tact of  others.  There  is  also  a  similar  observation  made  by  some  of  the  earher  chem- 
ists respecting  camphor.  Strange  motions  of  an  analogous  description  are  also  ob- 
served in  some  liquids  under  the  influence  of  a  voltaic  current ;  these,  in  the  case  of 
mercury,  have  been  particularly  studied  by  Sir  J.  Herschel,  who  obtained  several  re- 
markable notices  respecting  them ;  they  are,  however,  so  far  as  I  am  informed,  as  yet 
without  explanation. 

20.  If  into  a  watch-glass  or  shallow  capsule,  as  a  a  (Jig.  5,  pi.  1),  fifty  or  sixty 
grains  of  mercury  be  poured,  and  over  that  as  much  water,  acidulated  with  sulphuric 
acid,  as  is  sufficient  to  cover  the  surface  of  the  mercury,  and  the  positive  and  negative 
wires  of  a  battery  of  twenty  or  thirty  plates,  arranged  as  indicated,  the  mercury  being 
in  contact  with  the  negative  pole,  and  the  positive  pole  being  plunged  into  the  water 
at  a  short  distance  from  it,  currents  are  produced  both  in  the  water  and  in  the  mercury. 
Supposing  the  power  of  the  battery  sufficient,  the  same  effect  takes  place  on  removing 
the  negative  wire  out  of  the  mercury  into  the  water;  but  if  the  positive  wire  is  in  con- 
tact with  the  mercury,  and  the  negative  with  the  water,  there  is  no  motion  at  all,  or,  at 
most,  the  mercury  only  curls  itself  up  into  an  elongated  figure. 

21.  This  motion  varies  according  to  several  circumstances,  but  chiefly  from  the  po- 
sition of  the  two  wires.  1st.  If  the  wires  be  on  opposite  sides  of  the  mercury,  the  metal 
instantaneously  elongates,  and  currents  also  are  seen  playing  in  the  water.  2d.  If  the 
negative  wire  be  introduced  into  the  centre  of  the  metallic  globule,  and  the  positive 
wire  be  brought  on  one  side,  the  mercury  will  bulge  out  eUij)ticaUy  at  both  sides,  near- 
est and  farthest  from  the  positive  pole  ;  and  by  regulating  the  force  of  the  battery,  either 
by  changing  the  number  of  the  plates,  or  altering  the  strength  of  the  solution  acting  on 
them,  the  experiment  may  be  so  managed  that  no  motion  shall  ensue  in  the  mercury 


DESCRIPTION  OF  THE  PHENOMENON.  9 

after  this  elliptical  bulging  is  effected ;  but  now,  if  the  negative  wire  is  cautiously  raised 
from  its  position,  so  as  to  be  just  out  of  contact  with  the  surface  of  the  metal,  the  mer- 
cury is  immediately  convulsed,  and  its  whole  surface  covered  with  circular  waves.  On 
lowering  the  negative  wire  to  its  former  position,  and  advancing  the  positive,  the  mo- 
ment it  comes  to  the  edge  of  the  mercurial  ellipsoid,  the  most  intense  convulsions  are 
produced,  which  increase  until  contact  of  the  mercury  and  wire  is  obtained.  3d.  If  the 
two  wires  form  a  kind  of  triangle  with  the  globule,  it  turns  upon  itself 

22.  At  the  same  time  that  these  movements  are  going  on,  the  surface  of  the  water 
is  ploughed  by  gentle  currents,  exactly  resembling  those  produced  by  a  breath  from  a 
blowpipe,  directed  slantingly  across  the  surface. 

23.  In  proceeding  to  give  an  explanation  of  these  motions,  I  shall  not  follow  the  an- 
alytical course  of  experiment  used  in  my  researches,  but  commence  with  those  princi- 
ples on  which  a  true  explanation  is  founded. 

24.  It  has  long  been  known  that  the  elements  of  compound  substances  were  held 
together  in  virtue  of  an  affinity  among  themselves.  Sir  H.  Davy,  Berzehus,  and  other 
chemists,  were  led  to  suspect  that  this  was  due  to  the  electric  condition  of  those  ele- 
ments, and  pursuing  this  hypothesis  in  its  details,  several  brilliant  discoveries  were  made, 
which  ultimately  changed  the  face  of  the  science.  Apart,  however,  from  all  hypothet- 
ical reasoning,  it  was  found  that  the  poles  of  a  voltaic  battery  had  the  power  of  influ- 
encing the  atomic  constitution  of  bodies,  so  as  to  be  able  to  hold  all  chemical  combi- 
nation under  control.  This  remarkable  effect  was  imputed  to  the  electrical  attraction 
and  repulsion  of  the  battery ;  but  a  battery  which  is  competent  to  the  rapid  decompo- 
sition of  water,  and  even  the  reduction  of  potash,  is  found  to  give  exceedingly  faint 
traces  of  any  electro-dynamic  effect,  being  unable  to  cause  the  divergence  of  a  delicate 
gold  leaf  electrometer,  or  affect  the  indications  of  a  torsion  balance.  In  the  course  of 
certain  experiments,  I  had  occasion  to  notice  that  this  effect,  as  to  intensity,  is  entirely 
regulated  by  the  medium  in  which  the  experiment  is  made  ;  as,  for  instance,  a  thin 
lamina  of  air  or  gaseous  matter  is  nearly  a  perfect  non-conductor  to  electricity  of  low 
intensity,  but  a  mass  of  water  offers  no  such  resistance.  I  hoped,  therefore,  that  though 
I  might  not  be  able  to  exhibit  the  attraction  of  a  polar  wire  for  a  suspended  needle  in 
the  Coulomb  balance,  such  an  effect  might  ensue  if  the  experiment  was  made  with  the 
apparatus  plunged  in  another  atmosphere,  the  conducting  power  of  which  differed  from 
that  in  which  we  live.  For  the  conducting  power  of  a  medium  has  no  relation  either 
to  its  cohesion  or  its  chemical  properties,  and  it  did  not  appear  improbable  that  one 
might  be  found,  which,  though  it  should  not  interfere  with  the  freedom  of  motion  of  a 
wire  plunged  in  it,  its  conducting  power,  in  relation  to  electricities  of  very  low  in- 
tensity, might  exhibit  those  effects  in  a  more  elevated  point  of  view. 

25.  To  illustrate  this  reasoning,  I  took  a  platina  wire,  a  c  {Jig.  6,  pi.  1),  two  inches 
in  length,  and  suspended  it  by  a  raw  silk  thread  from  a  stand,  h  h,  into  a  vessel  filled 
with  acidulated  water,  as  high  as  d  d.  The  needle  was  so  arranged  that  when  it  hung 
with  freedom,  it  was  about  one  fourth  of  an  inch  distant  from  the  extremities  of  two  plati- 
na-pointed  wires,  p  n,  which  entered  the  vessel  on  opposite  sides,  and  could  be  made  to 
communicate  at  will  with  the  opposite  poles  of  a  battery.    Now  the  wire  p  being  pos- 

B 


10 


THE  POLAR  WIRES  ACT  AS  CENTRES  OF  ATTRACTION. 


itive,  and  n  negative,  the  extremity  a  of  the  suspended  needle  would  he  negative,  and 
the  extremity  c  positive  by  induction.  The  conjoined  effort  of  the  forces  thus  brought 
to  bear  on  the  needle,  acting  on  its  opposite  extremities  in  opposite  directions,  would 
solicit  it  to  move  on  its  axis,  the  extremity  a  in  the  direction  n'  {Jig  7),  and  the  ex- 
tremity c  in  the  direction  p' ;  the  line  of  rest  would  be  as  expressed  by  the  dots  in  the 
figure,  and  slow  oscillations  should  take  place  on  either  side  of  that  line  if  the  density 
or  other  properties  of  the  medium  permitted. 

26.  The  experiment  was  thus  tried,  and,  to  prevent  any  derangement  from  hygro- 
mctric  twist  of  the  silk,  the  needle  was  hung  on  a  glass  thread,  of  sufficient  length  to 
reach  above  the  surface  of  the  water,  and  there  attached  to  the  silk ;  on  passing  the 
current  of  forty-five  pairs  of  four  inch  plates,  the  needle  immediately  moved,  and  after 
two  or  three  oscillations,  took  its  position  of  rest ;  on  being  moved  to  the  opposite  side 
of  the  polar  wire,  an  opposite  motion  ensued  until  the  same  position  was  gained.  Du- 
ring this  movement,  gas  was  freely  liberated  from  the  extremities  of  the  polar  wires, 
and  also  from  both  ends  of  the  needle,  it  hindered  considerably  that  freedom  of  mo- 
tion which  I  had  hoped  for  in  observing  the  oscillations.    The  experiment  was  also 
varied  by  terminating  the  polar  wires  with  plates  of  platina,  with  a  view  of  increasing 
the  effect ;  the  needle  was  also  suspended  in  pyroligneous  ether,  and  the  attractive  pow- 
er of  the  same  battery,  newly  charged,  was  very  marked ;  it  was  not  so  observable  in 
alcohol,  and  still  less  in  muriatic  acid ;  in  ammonia,  though  only  one  end  of  the  needle 
appeared  to  evolve  gas,  it  was  not  so  obedient  to  the  attractive  force.    These  cir- 
cumstances indicate  that  the  phenomena  of  motion,  as  here  exhibited,  have  not  their 
origin  in  any  magnetic  action  produced  either  by  the  disturbance  of  the  earth  or  the 
passage  of  the  voltaic  currents.    Magnetic  action,  to  be  complete,  requires  that  the  bod- 
ies along  which  currents  are  passing  should  be  possessed  of  high  conducting  power ; 
hence  a  thermal  current,  whose  tension  is  almost  extinct,  is  still  capable  of  producing  a 
powerful  effect  on  a  suspended  needle.    A  current  capable  of  producing  a  given  devia- 
tion when  moving  along  metalhc  wires,  woiild  meet  with  resistance  in  passing  through 
water;  and  alcohol  or  ether  would  forbid  its  passage.    It  is,  moreover,  impossible  to 
produce  any  visible  effect  on  the  platina  wire  of  this  arrangement  by  the  action  of  a 
single  pair,  even  possessing  extensive  surface,  though  the  same  pair,  if  cut  into  lesser 
plates,  and  arranged  for  the  production  of  a  current  of  greater  tension,  immediately 
causes  the  movement  here  described.    Dr.  Faraday  has  stated,  in  his  recent  researches 
on  this  point,  that  there  is  not  any  proof  that  the  poles  of  a  battery  do  exercise  any 
power  of  attraction  or  repulsion  {Baches  Turners  Che?n.,  p.  102  ;  idem,  108)  ;  but  that 
opinion  would  appear  to  be  inconsistent  with  the  fact — there  must  be  an  accumulation 
of  tension  on  an  electrode,  if  the  medium  which  separates  it  from  its  fellow  is  not  so 
good  a  conductor  as  the  liquid  filling  the  cells  of  the  battery ;  and  experiment  warrants 
this  conclusion. 

27.  The  principles  here  laid  down  also  indicate  the  construction  of  a  galvanometer 
which  !  have  recently  fitted  up.  It  is  intended  to  exhibit,  by  the  torsion  of  a  fine  fibre, 
the  force  of  attraction  between  the  polar  wires  and  the  ends  of  the  suspended  needle. 
The  obstacle  I  have  observed  to  the  accuracy  of  the  results  furnished  by  it,  is  due  to  the 
development  of  gas  on  the  polar  wires  and  on  the  needle. 


THEY  PRODUCE  TIDES.  H 

28.  The  doctrine  which  I  wish  to  estabUsh  from  this  experiment  is,  that  though  the 
polar  wires  are  plunged  in  a  conducting  medium,  and  the  current  is  actually  passing, 
yet  they  still  act  as  centres  of  attraction.  The  motions  of  mercury  and  other  fluids  are 
only  exemplifications  of  this  doctrine. 

29.  When  a  spheroidal  mass  of  conducting  matter  is  brought  in  presence  of  a  point 
of  attraction,  situated  at  a  distance  from  its  surface,  the  particles  on  that  surface  will  be 
differently  affected  as  their  situation  in  regard  to  the  attracting  point  varies.  Thus,  on 
touching  the  mercurial  globule,  named  in  the  first  part  of  this  paper,  with  a  negative 
wire,  and  introducing  into  the  water  a  positive  platina  pole,  the  globule,  which  before 
was  spherical,  becomes  ellipsoidal,  two  tides  are  formed  upon  it,  one  directly  opposite 
the  positive  wire,  and  the  other  180  degrees  from  it ;  meanwhile  there  is  an  ebb  in  those 
regions  which  are  situate  a  quadrant  from  the  point  of  attraction.  If  the  positive  wire 
is  made  to  revolve  round  the  globule,  both  tides  move,  always  keeping  the  same  rela- 
tive position  to  the  point  of  attraction  that  they  had  at  first.  It  only  requires  the  force 
of  the  battery  to  be  appropriately  moderated  to  exhibit  these  phenomena  with  the  ut- 
most rigidness.  And  as  these  motions  exhibit  very  nearly,  on  a  small  scale,  that  effect 
which  takes  place  on  an  immense  scale  by  the  joint  action  of  the  Sun  and  Moon  in 
producing  the  tides  of  the  ocean,  I  have  given  them  the  name  of  Tidal  motions  of  mo- 
vable Conductors. 

30.  Now  the  mechanism  which  produces  the  change  of  figure  from  a  sphere  to  an 
ellipsoid  is  sufficiently  obvious.  We  have  two  forces  under  consideration  :  1st.  The 
cohesion  or  gravitation  of  the  mercurial  particles  upon  each  other ;  and,  2d.  The  dis- 
turbing force  of  the  polar  wire  as  a  centre  of  attraction.  As  that  disturbing  force  de- 
creases in  a  certain  ratio,  as  the  distances  increase,  the  mercurial  particles  on  the  side, 
A  {fig.  8,  pi.  1),  nearest  to  the  polar  wire  are  more  attracted  by  it  than  those  in  the  cen- 
tre, C,  of  the  globule,  and  those  in  the  centre,  C,  are  more  attracted  than  those  at  F. 
The  particles,  therefore,  at  A  rise  towards  the  wire  by  its  direct  action,  those  at  F  being 
less  solicited  towards  the  centre  of  the  globule  than  those  at  E  and  B  ;  the  former  re- 
cede from  that  centre,  while  the  latter  seek  it. 

31.  It  has  been  observed  that  a  true  theoretical  tide  differs  in  no  respect  from  a  wave- 
"  Suppose  a  spring  tide  actually  formed  on  a  fluid  sphere,  and  the  sun  and  moon  then 
annihilated,  the  elevation  must  sink,  pressing  the  under  waters  aside,  and  causing  them 
to  rise  where  they  were  depressed.  The  motion  will  not  stop  when  the  surface  comes 
to  a  level,  for  the  waters  arrive  at  that  position  with  a  motion  continually  accelerated. 
They  therefore  pass  that  position,  as  a  pendulum  passes  the  perpendicular,  and  will 
rise  as  far  on  the  other  side,  forming  a  high  water  where  it  was  low  water,  and  low 
water  where  it  was  high  water.  And  this  would  go  on  forever,  oscillating  in  an  as- 
signable time,  if  it  were  not  for  the  viscidity  of  the  water."  Now  this  theoretical  case 
may  be  easily  shown,  for  on  approaching  the  positive  wire  towards  the  globule  of  mer- 
cury, a  particular  position  will  be  gained,  at  which  contact  will  take  place  between  the 
protuberant  tide  on  the  mercury  and  the  wire.  In  that  moment  the  cause  of  attraction 
is  annihilated,  the  whole  current  of  electricity  now  passes  along  perfect  conductors, 
hence  fulfilling  the  supposed  case  of  an  actual  annihilation  of  the  sun  and  moon  at  the 
time  of  spring  tide.    And  the  same  reasoning  that  held  in  one  case,  equally  applies  in 


12 


CAUSE  OF  THE  SPIRAL  MOTIONS. 


the  Other ;  the  mercurial  tide  falls  with  an  accelerated  motion,  and  the  line  which  before 
was  the  conjugate  axis  of  the  ellipse,  now  becomes  the  transverse,  a  tide  being  pro- 
duced at  right  angles  to  the  former  one.  But  here  the  strict  comparison  ends,  for,  as 
the  mercury  ebbs  from  its  protuberant  position,  the  metallic  connexion  breaks,  and  the 
wire  is  again  put  in  action  as  a  point  of  attraction  ;  the  motion  of  the  ebbing  tide  is 
checked  ;  it  flows  once  more  ;  once  more  the  metallic  contact  is  complete,  and  when 
the  tide  falls,  it  is  only  to  flow  again,  as  long  as  the  battery  current  passes.  Tides  take 
place  at  right  angles  to  each  other,  in  a  series  too  rapid  to  be  counted,  and  the  whole 
surface  of  the  mercury  is  worked  into  those  various  and  beautiful  undulations  which 
have  been  before  referred  to. 

32.  In  endeavouring  to  ascertain  the  true  cause  of  these  phenomena,  the  French  phi- 
losophers were,  I  believe,  the  first  to  observe  motions  in  the  water  or  other  liquid  of 
communication,  as  if  a  gentle  wind  played  over  its  surface,  bearing  light  bodies  in  its 
vortex.  The  explanation  of  these  appearances  I  here  add,  because  no  one  as  yet  has 
given  it,  and  it  affords  an  illustration  of  certain  propositions  dehvered  by  Sir  I.  Newton, 
in  his  Principia,  concerning  the  doctrine  of  pulses  in  elastic  fluids. 

33.  We  have  hitherto  been  considering  a  globule  of  mercury  as  a  substance  mathe- 
matically fluid.  Such,  however,  in  effect,  it  is  not ;  the  water  in  contact  with  it  pos- 
sesses those  properties  in  a  much  more  eminent  degree,  so  that,  in  comparison  with  it, 
the  mercury  may  be  regarded  as  a  solid  resisting  obstacle.  Now,  about  a  year  ago,  I 
showed  that  when  a  voltaic  current  passes  through  a  system  such  as  this  of  mercury 
and  water,  the  capillary  pressure  on  the  bounding  surface  is  changed  ;  but,  if  the  at- 
traction of  the  wire  which  is  introduced  into  the  water,  and  which  is  the  ultimate  cause 
of  this  derangement,  decreases  in  a  duplicate  ratio,  it  follows  that  this  disturbance  ot 
pressure  obtains  only  to  a  limited  extent  on  the  surface  of  the  mercury  ;  or,  in  other  words, 
the  excess  of  pressure  produced  by  a  voltaic  current  is  not  spent  equally  on  all  parts  of 
the  mercurial  surface,  but  those  which  are  adjacent  to  the  positive  polar  wire  are  more 
affected  than  those  at  a  distance.  Newton  has  shown  {Principia,  v.  ii.,  b.  ii.,  pr.  41), 
that  if  the  particles  of  a  fluid  do  not  lie  in  a  right  line,  a  pressure  propagated  through 
that  fluid  will  not  be  in  a  rectilineal  direction,  but  the  particles  that  are  obliquely  posited 
have  a  tendency  to  be  urged  out  of  their  position.  So  the  particles  a  a  aa  {fig.  9,2^1. 
1),  pressing  on  the  particles  d  b,  which  stand  obliquely  to  them  by  reason  of  the  shape 
of  the  mass  of  mercury  g,  have  a  tendency  to  be  urged  from  their  places  towards  e 
and  c  respectively,  and  the  motion  thus  produced  in  a  fluid  diverges  from  a  rectilinear 
progress  into  the  unmoved  spaces ;  and  such  a  pressure  taking  effect  on  a  liquid  free  to 
move,  continually  returns  the  moving  particles  to  their  first  position,  after  making  them 
describe  an  elliptical  orbit. 

34.  It  has  been  remarked  that  the  basis  on  which  this  explanation  essentially  rests 
is,  that  a  wire,  from  which  an  electric  current  passes,  acts  still  as  a  point  of  attraction ; 
an  effect  which  involves  the  conducting  and  other  electric  properties  of  the  system  on 
which  the  experiment  is  tried.  Hence  we  gain  an  insight  into  the  cause  of  the  paral- 
ysis of  these  motions  by  the  addition  of  certain  substances  ;  the  spiral  motions  going  on 
over  the  surface  of  the  water  have  these  explanations  complicated  with  another  con- 
sideration, the  figure  of  the  mercurial  mass. 


OF  THE  MODE  OF  PASSAGE  OF  LIQUIDS  THROUGH  PORES.  X3 


CHAPTER  III. 

ON  THE  INTERSTITIAL  MOVEMENTS  WHICH  TAKE  PLACE  AMONG  THE  PARTICLES  OF  BODIES. 

{Frora  the  Journal  of  the  Franklin  Institute  for  March  and  July,  1836.) 

Contents  :  Of  the  Mode  of  Passage  of  Liquids  through  Pores. — Endosmosis. — Perco- 
lation through  Gum  Lac,  Gold  Leaf,  Mica,  etc. — Sloio  Motions  in  the  Parts  of  Solid 
Bodies,  as  in  Silver  Coins. — Percolation  through  Lidia  Rubber. — Conditions  of  Equ  i- 
librium.— Percolation  through  Masses  of  Water. — Percolation  through  excessively  thin, 
Films  of  Water,  as  Soap  Bubbles. — Analysis  of  Gas  on  the  Exterior  and  in  the  Inte- 
rior of  the  Soap  Bubble. — General  Law  of  the  Phenomenon  deduced. 

35.  The  interstices  wliich  exist  in  a  great  variety  of  bodies  may  be  looked  upon  as  an 
extensive  system  of  capillary  tubes,  into  which  we  should  be  prepared  to  expect  that 
bodies  of  all  kinds  might  pass.  A  drop  of  water  placed  upon  a  porous  stone  or  a  piece 
of  chalk,  sinks  into  it  rapidly,  but  the  value  of  the  observation  is  lost,  because  it  is  com- 
mon. If  that  water  contained  a  colouring  matter,  we  should  find  that,  in  sinking  into 
the  chalk,  the  colour  would  be  left  on  the  surface.  But  here,  again,  commonplace  princ  i- 
ples  dictate  a  ready  answer :  the  interstices  of  the  chalk  may  be  supposed  to  be  too  small 
to  admit  the  colouring  matter  to  pass,  or  perhaps  some  incongruity  of  shape  might  afford  a 
barrier ;  yet  how,  upon  these  principles,  shall  we  explain  that  mercury  and  other  bodies 
remain  unmoved  upon  the  porous  mass,  and  show  no  ability  to  go  through  it,  when  they 
will  pass  with  readiness  into  the  densest  and  closest  substances,  as  gold  \  No  principle 
of  coaptation  will  explain  why  quicksilver  will  not  rise  in  a  tube  of  glass,  or  why  water 
rises  at  all.  We  are  induced  at  once  to  refer  the  whole  matter  to  the  chemical  condi- 
tions of  the  bodies  on  which  we  operate,  and  we  quickly  infer  that  fluids  do  not  pass 
into  pores  by  soaking  or  leakage,  or  any  such  commonplace  principle,  but  that  it  is  an 
action  determined  by  certain  laws  that  have  reference  to  the  condition  of  each  body  sep- 
arately, and  their  relation  to  each  other.  A  question,  therefore,  naturally  arises  as  to  the 
peculiar  operation  of  those  pores,  and  how  changes  in  their  position,  size,  and  shape 
affect  the  results  of  their  action.  A  class  of  these  phenomena  is  quite  independent  of 
pores  of  any  sensible  size,  where  no  leakage  or  oozing  can  be  suspected.  A  piece  of 
sugar  dissolving  in  water  diffuses  itself  into  every  part  of  the  menstruum.  Among  those 
excessively  small  interstices  that  exist  between  the  atoms  of  the  water,  its  particles  find 
a  dweUing,  where  they  are  sheltered  from  all  those  forces  that  act  so  energetically  on 
the  great  masses  of  matter.  Independent  of  gravity,  they  move  freely  in  every  direction  ; 
and,  far  from  settling  in  those  positions  to  which  they  might  tend  from  their  weight,  they 
are  simultaneously  and  equally  found  in  every  portion  of  the  solvent.  This  condition 
of  things  does  not  indicate  a  passive  state,  but  would  rather  teach  that  a  very  active  and 
powerful  force  is  in  operation,  a  force  that  can  neutralize  the  action  of  gravity  and  other 
external  agents.     It  is  essential,  therefore,  clearly  to  understand  the  circumstances  of 


14 


OF  ENDOSMOSIS. 


this  absorption ;  it  may  take  place  independently  of  apertures,  pores,  or  vessels  ;  it  may 
take  place  between  gases  and  gases,  gases  and  vapours,  or  liquids,  or  solids,  or  mutually 
and  indiscriminately  among  them  all. 

36.  When  a  liquid  rises  in  a  capillary  tube,  those  portions  only  are  under  the  direct 
influence  of  the  attractive  force  of  the  tube  vi^hich  are  nearest  to  it,  the  central  columns 
being  entirely  unaffected.  Also,  when  water  jets  out  through  a  narrow  pipe,  it  is  only 
those  portions  that  are  directly  in  contact  with  the  sides  of  the  pipe  that  are  subject  to 
its  resisting  influences,  any  disturbance  which  the  central  particles  feel  arising  indi- 
rectly from  their  cohesion.  The  same  applies  in  the  passage  of  liquids  among  pores: 
the  diameter  of  these  pores  amounting  to  a  certain  size,  they  will  admit  a  passage  with- 
out exerting  any  direct  influence.  Thus,  a  pore  in  a  piece  of  charcoal  may  suffer  a 
column  of  water  to  go  through  it,  without  in  any  wise  affecting  the  central  portion  of 
that  column,  by  reason  of  its  size  ;  but  should  the  diameter  of  the  pore  be  made  to  de- 
crease, it  is  obvious  a  limit  might  finally  be  reached,  where  every  particle  that  passed 
should  come  under  the  direct  influence  of  the  physical  force  exerted  by  the  pore,  and 
none  pass  by  mere  leakage  or  oozing. 

37.  This  leads  us  to  consider  the  different  effects  that  may  ensue  when  the  same 
liquid  or  gas  passes  through  pores  of  various  sizes  in  the  same  solid.  An  example  may, 
perhaps,  illustrate  the  results  :  The  walls  of  a  pore  are  so  constituted  as  to  allow  an 
easy  passage  of  one  gas,  as  oxygen,  along  them,  and  afford  more  or  less  resistance  to 
another,  as  nitrogen  gas.  Now,  if  we  suppose  this  pore  to  be  of  very  large  size,  and 
atmospheric  air  to  be  passing  through  it,  little  or  no  change  will  happen  in  the  consti- 
tution of  the  passing  gas,  all  the  internal  parts  of  the  current  being  out  of  the  reach  of 
the  walls  of  the  pore ;  but  should  the  diameter  of  the  pore  be  reduced  to  the  diameter 
of  an  atom  of  the  compound  gas,  or  thereabout,  the  oxygen,  finding  little  or  no  resist- 
ance, would  glide  through,  and  the  nitrogen  be  retained,  a  perfect  decomposition  happen- 
ing. This  shows  the  importance  in  all  investigations  relative  to  Endosmosis,  or  transit 
of  bodies  through  pores,  of  bearing  in  mind  that,  when  those  pores  have  a  certain  di- 
ameter, the  results  of  experiments  made  on  them  are  illusive,  not  representing  alone  the 
nature  and  value  of  the  force  exerted  by  the  walls  of  the  pore,  but  showing  effects  de- 
pending also  on  the  cohesion  and  other  properties  of  the  passing  body. 

38.  These  observations  apply  to  those  experiments  which  have  been  made  to  illus- 
trate the  phenomena  of  endosmosis  by  forcing  gases  through  plugs  of  stucco,  which  are 
systems  of  capillary  tubes  of  large  size.  Experiments  on  charcoal,  plaster,  &c.,  are  also 
open  to  the  same  strictures.  Had  these  only  been  resorted  to,  the  simplest  phenomena 
of  endosmosis  could  not  have  been  discovered.  The  disturbance  of  hydrostatic  level, 
which  is  so  well  shown  by  a  sheet  of  gum  clastic,  or  an  animal  membrane,  cannot  be 
produced  by  the  use  of  plugs  with  large  pores  or  systems  of  capillary  tubes. 

39.  It  might  at  first  be  expected  that,  as  the  diameter  of  a  pore  decreased,  its  indis- 
position to  admit  a  foreign  body  would  increase ;  but  it  is  not  so :  that  foreign  atom 
does  not  insinuate  itself  in  a  passive  manner,  nor  does  it  go  through  the  pore  merely  be- 
cause it  meets  with  no  resistance.  There  is  an  active  and  very  energetic  force  in  play, 
a  force  that  is  even  greater  than  the  cohesion  of  the  parts  of  the  pore  itself.  Hence, 


/ 

PERCOLATION  THROUGH  GUM  LAC,  ETC.— SLOW  MOTION  IN  THK  PARTS  OF  SOLIDS.  25 

under  like  circumstances,  the  smallness  of  such  a  pore  is  no  bar  to  its  receiving  and 
transmitting  foreign  atoms,  but  very  often,  in  an  experimental  point  of  view,  is  the  most 
favourable  condition  under  which  we  can  study  its  action  without  any  retarding  or 
comjdox  causes. 

40.  There  are  some  experimental  illustrations  of  the  fact,  that  closeness  of  texture  is  110 
hinderance  to  the  passage  of  suitable  bodies.  I  took  a  narrow  glass  pipe,  about  an  eightli 
of  an  inch  in  diameter,  and  dipping  one  end  of  it  into  melted  gum  lac,  expanded  there- 
on a  bul)ble  of  that  substance,  by  blowing  at  the  other  extremity.  In  this  way,  after  a 
few  trials,  the  bubble  may  be  made  so  thin  as  to  be  translucent.  Such  a  bubble,  with 
air  from  the  lun^s  in  its  interior,  being  exposed  to  an  atmosphere  of  ammoniacal  gas, 
allows  a  free  passage  to  it.  A  singular  change  in  the  appearance  of  the  thin  membranous 
bag  takes  place  during  the  experiment:  from  being  brown  in  the  thicker  parts  and  whitish 
in  those  that  are  more  translucent,  it  becomes  of  one  uniform  flesh  colour.  Now  in  this 
state  it  may  be  regarded  as  one  of  the  most  impervious  of  all  resinous  bodies,  and  cer- 
tainly of  them  all  it  has  the  closest  texture ;  yet,  after  it  has  thus  been  exposed  for 
a  short  time  to  ammonia,  we  find,  on  passing  into  its  interior  a  little  reddened  litmus 
water,  that  the  gas  is  present  in  large  quantitv,  and  must,  of  course,  have  been  trans- 
mitted along  the  pores  in  the  resin. 

41.  On  the  top  of  a  tube  which  contained  atmospheric  air  and  apiece  of  litmus  pa- 
per, tinged  red  by  the  fumes  of  muriatic  acid,  I  fastened  very  carefully  a  piece  of  gold 
leaf,  two  tenths  of  an  inch  in  diameter,  with  gum-water,  and  suffered  it  to  dry.  Tlie 
gold  leaf,  when  examined  with  a  lens  by  transmitted  light,  appeared  all  over  of  a  uni- 
form pea-green  colour,  nor  could  any  hole  or  flaw  be  perceived  in  it.  It  was  covered  \\  itb 
ajar  of  ammonia  on  the  mercurial  trough,  the  level  of  the  mercury  on  the  inside  and  the 
outside  being  regulated.  The  gas  went  through  the  gold  leaf  rapidly,  and  in  a  very 
short  time  the  test  paper  became  uniformly  blue.  On  using  carbonic  acid  or  sulphu- 
retted hydrogen,  the  action  was  very  nearly  as  instantaneous. 

42.  I  split  with  a  lancet  a  thin  plate  of  Siberian  mica,  which  for  the  most  part  ap- 
peared of  a  flame  colour,  but  in  places  where  it  was  unequally  thick,  a  blue  or  a  red. 
This  plate,  when  substituted  for  gold  leaf  in  the  last  experiment,  suffered  anunonia  to  pass 
through  it.  A  similar  plate  of  sulphate  of  lime  suffered  half  a  cubic  inch  of  carbonic 
acid  to  pass  through  it  in  forty  minutes.  Atmospheric  air,  in  all  these  cases,  was  on  the 
other  side. 

43.  These  permeations,  which  we  have  noticed  to  take  place  so  rapidly  under  favour- 
able circumstances,  occur  likewise  more  slowly  in  nature.  A  sea-shell,  for  instance,  de- 
posited in  that  formation  called  Loudon  clay,  in  course  of  time  loses  its  coagulated  albu- 
men, then  its  carbonate  of  lime,  and  its  other  ingredients  simultaneously  or  successively. 
These  are  replaced  by  the  sulphuret  of  iron,  by  alumina,  oxide  of  iron,  &c.,  which  form 
together  a  mass  of  so  close  a  texture,  that  it  can  give  sparks  by  collision.  Under  such 
circumstances  as  those  which  occur  along  the  coast  of  the  Island  of  Sheppey,  a  thiti 
plate  of  carbonate  of  Ume  is  permeated  readily  by  bisulphuret  of  iron,  so  thai  there  is  a 
continued  deposition  and  accumulation  of  that  substance,  even  in  the  interior  of  a  thin 
shell.    Hence  the  production  of  that  immense  quantity  of  fossil  shells,  which  is  there 


16 


PERCOLATION  THROUGH  INDIA-RUBBER.— CONDITION  OF  EQUILIBRIUM. 


used  for  the  purpose  of  manufacturing  copperas  for  commerce.  Slow  motions  of  the 
same  kind  occur  when  alloys  arc  buried  under  the  ground,  or  placed  in  exposed  situa- 
tions ;  a  silver  Roman  coin  has  thus  been  known  to  part  with  much  of  its  copper,  which 
formed  a  species  of  crystallization  on  its  surface,  the  patina  of  antiquarians.  It  is  in 
this  way  that  trinkets  of  gold,  on  which  small  quantities  of  mercury  have  fallen,  gradu- 
ally recover  their  original  brilliancy  and  purity.  A  number  of  facts  of  this  kind,  show- 
ing that  even  in  the  most  solid  of  metallic  textures  motions  may  take  place,  might  be 
referred  to :  these  have  been  well  considered  by  Boyle,  in  his  tract  on  the  languid  mo- 
tions of  bodies. 

44.  Caoutchouc,  or  gum  elastic,  is  the  substance  which,  of  all  others,  has  furnished  the 
most  unexceptionable  results  on  studying  the  phenomena  of  endosmosis.  It,  however, 
at  times  exerts  a  synthetic  action,  which,  so  far  as  I  know,  has  not  yet  been  noticed. 
Having  capped  an  open  tube  with  a  thin  piece  of  this  substance,  and  thrown  into  it 
200  measures  of  hydrogen  gas,  it  was  exposed  to  an  atmosphere  of  100  measures  of 
oxygen  contained  in  a  wider  tube,  into  which  it  was  raised.  In  eleven  days  the  level 
in  both  tubes  had  considerably  risen,  and  the  barrier,  which  was  at  first  of  a  blackish 
colour,  became  quite  white.  In  sixteen  days,  the  united  volume  of  both  gases  was  only 
215  measures;  this,  on  analysis,  contained  only  14  per  cent,  of  oxygen.  It  may  here 
be  stated  that  a  like  mixture  of  100  of  oxygen  and  200  of  hydrogen,  enclosed  together 
in  a  tube  by  the  side  of  the  former,  had  undergone  little  or  no  diminution.  Now  a 
rough  calculation  shows  that  about  one  thirteenth  of  the  united  volume  of  the  gases  haxl 
been  condensed  by  the  membrane  into  water,  the  remaining  02-35  parts  of  oxygen  hav- 
ing combined,  in  some  chemical  manner,  with  the  substance  of  the  caoutchouc,  in  the 
process  of  bleaching  it. 

45.  This  result  points  out  the  condensing  effect  of  a  membrane,  which  often,  in  many 
arrangements,  will  have  no  small  influence.  Thus,  in  Dr.  Mitchell's  experiment,  where 
two  bent  tubes  are  screwed  together,  with  a  piece  of  gum  elastic  between  them,  the  one 
tube  containing  oxygen,  and  the  other  a  double  volume  of  hydrogen,  we  should  be  led 
to  expect,  from  the  common  theory  of  endosmosis,  that  however  much  the  levels  in  the 
two  tubes  might  vary  relatively  to  each  other,  the  united  volume  of  the  gases  ought  to 
remain  constant.  If  the  level  in  the  hydrogen  tube  rose  an  inch,  ought  not  the  level  in 
the  oxygen  tube  to  sink  an  inch?  But  an  appeal  to  experiment  shows  that  such  is  by 
no  means  the  fact ;  to  a  certain  extent,  the  volume  of  gas  in  the  tubes  is  constantly  di- 
minishing ;  it  is  not  due  to  leakage  into  the  free  atmosphere,  between  the  membrane  and 
the  glass  that  presses  it,  at  least  not  entirely  so ;  for  a  part  of  the  gases  is  condensed  by 
the  direct  action  of  the  barrier  to  form  water,  and  the  remainder  unites  ciiemically  with 
it.  In  some  instances,  the  action  is  still  more  obvious.  If  a  vessel  of  atmospheric  air, 
the  mouth  of  which  is  covered  by  a  piece  of  India-rubber,  be  immersed  in  an  atmo- 
sphere of  deutoxide  of  nitrogen,  it  will  be  found  that  red  fumes  do  not  appear  in  the 
vessel,  nor  any  other  obvious  indication  of  the  presence  of  the  deutoxide,  but  the  mem- 
brane soon  begins  to  change  its  colour,  and  from  being  diaphanous,  becomes  of  a  dirty 
ninber  brown,  the  volumes  of  the  gases  on  both  sides  of  it  diminishing. 

If).  Into  a  tube  which  was  covered  with  India-rubber,  standing  on  the  shelf  of  the 


1 


CONDITION  OF  EQUILIBRIUM. 

pneumatic  trough  and  exposed  to  the  free  atmosphere,  I  placed  100  measures  of  atmo- 
spheric air  and  42  of  hydrogen  gas,  being  anxious  to  see  if  any  passage  of  the  gases 
would  ensue,  as  the  oxygen  and  hydrogen  in  the  mixture  were  in  nearly  a  due  propor- 
tion to  form  water.  Motion  at  once  began,  the  level  of  the  water  in  the  tube  rising  for 
several  hours.  In  the  course  of  a  few  days,  only  a  trace  of  hydrogen  was  discoverable, 
the  remaining  gas  differing  very  slightly  from  atmospheric  air.  The  same  was  repeated 
with  a  tube  closed  by  a  serous  membrane,  kept  continually  moistened;  when  all  motion 
appeared  at  an  end,  analysis  showed  that  there  was  only  j^th  of  the  whole  volume  of 
hydrogen  beneath  the  membrane. 

47.  These  experiments,  which  were  repeated  again  and  again  with  the  same  results, 
establish  an  important  doctrine.  If  a  gas  be  confined  beneath  a  system  of  pores,  the 
other  extremity  of  which  communicates  with  another  gas,  movement  will  ensue,  until 
the  constitution  of  the  gas  on  both  sides  of  the  system  is  alike.  If  oxygen  and  hydro- 
gen be  thus  placed,  they  will  mutually  pass  to  each  other,  nor  will  that  motion  cease 
until  the  resulting  compound  on  both  sides  of  the  membrane  is  the  same  chemically. 
This  endeavour  to  an  equalization  of  constitution  takes  place  under  all  circumstances  ; 
it  may,  perhaps,  be  partially  arrested  by  the  condensing  action  of  the  barrier.  There 
are,  therefore,  two  prominent  conditions  under  which  the  phenomena  of  endosmosis 
may  be  regarded:  1st.  During  the  state  of  motion.    2d.  After  an  equilibrium  is  obtained. 

48.  Aided  by  this  principle,  we  can  explain  how  mixtures  of  gases  would  comport 
themselves  when  exposed  to  free  atmospheres,  or  when  shut  up  in  close  chambers.  The 
arrangement  of  (46)  will  serve  as  an  illustration  :  here  we  have  a  mixture  of  atmospheric 
air  and  hydrogen  exposed  to  the  free  atmosphere.  It  is  evident  that,  in  pursuance  of 
an  attempt  to  gain  an  equilibrium,  a  portion  of  air  from  the  atmosphere  should  pass  in- 
ward through  the  membrane,  and  a  portion  of  hydrogen  pass  out.  But  as  soon  as  the 
hydrogen  is  beyond  the  outside  of  the  membrane,  it  is  dissipated  by  aerial  currents,  or 
otherwise  diffused  in  the  mass  of  the  atmosphere,  the  condition  of  equilibrium  being  in 
nowise  approached  to,  for  so  fast  as  the  hydrogen  escapes,  it  is  carried  off ;  there  be- 
ing continually  hydrogen  and  atmospheric  air  on  one  side  of  the  membrane,  and  only  at- 
mospheric air  on  the  other.  Equilibrium,  therefore,  can  only  be  gained  by  the  entire 
dissipation  of  the  hydrogen  into  the  free  air,  and,  accordingly,  experiment  indicates  that 
when  that  equilibrium  is  gained,  the  hydrogen  has  vanished,  and  atmospheric  air  is 
found  on  both  sides  of  the  membrane.  But  very  different  would  that  action  be  if  the 
arrangements  were  included  in  a  close  chamber,  as  beneath  a  small  glass  bell ;  here, 
when  the  hydrogen  comes  out  through  the  membrane,  it  does  not  escape,  but  continu- 
ally accumulates,  and  motion  ceases,  and  equilibrium  is  gained  when  the  relative  pro- 
portion of  the  gases  outside  the  membrane  is  the  same  as  inside.  Hydrogen,  therefore, 
in  this  case,  is  found  on  both  sides  of  the  barrier. 

49.  Before  proceeding  to  give  an  account  of  the  chemical  changes  that  may  happen  in 
virtue  of  the  action  of  capillary  forces,  it  is  necessary  to  remark,  that  all  the  analyses  of 
gaseous  mixtures,  in  which  oxygen  is  an  element,  have  been  uniformly  made  by  means 
of  binoxide  of  nitrogen.  Living  in  a  climate  where  no  dependance  can  be  placed  on  the 
action  of  an  electrical  machine,  and  not  possessing  Dr.  Hare's  galvano-ignition  apparatus, 

C 


18 


CONDITION  OF  EQUILIBRIUM. 


I  was  led  by  necessity  to  choose  between  spongy  platina  and  the  binoxide  of  nitrogen 
After  an  experience  of  some  extent  in  the  employment  of  this  gas,  it  has  not  appeared 
to  deceive  me  ;  it  is,  indeed,  an  chgible  method  in  gaseous  analysis  where  oxygen  is  con- 
cerned. The  mode  of  manipulation  is  as  follows  :  with  the  sliding  rod  eudiometer,  throw 
100  measures  of  the  gas  under  trial  above  the  surface  of  water  that  has  been  duly  ex- 
posed to  the  atmosphere,  and  contained  in  an  inverted  bell,  rather  wide  in  proportion  to 
its  depth;  one  made  of  the  belly  of  a  glass  retort  or  a  cupping-glass  answers  very  well. 
Then  add  100  measures  of  the  binoxide  of  nitrogen  if  the  gas  is  suspected  to  be  poor 
in  oxygen,  but  200  or  more  if  the  gas  is  richer,  always  observing  to  have  the  binoxide 
in  excess.  After  the  lapse  of  a  minute  the  absorption  is  complete  ;  measure  the  resi- 
due, and  one  fourth  of  the  diminution  gives  the  volume  of  oxygen:  this  method  is  anal- 
ogous to  that  of  Gay  Lussac.  Some  idea  of  its  correctness  may  be  formed  from  the  cir- 
cumstance that,  of  73  analyses  of  the  air,  the  mean  result  of  the  amount  of  oxygen  is 
20-58  per  cent.  My  measuring  rod  divides  each  volume  into  decimals  by  a  vernier  ar- 
rangement ;  but  for  most  purposes  of  analysis  this  is  unnecessary. 

50.  It  results,  from  the  observations  which  have  been  made  on  caoutchouc  by  Dr. 
Mitchell,  that  oxygen  passes  through  it  with  much  more  facility  than  nitrogen.  Atmo- 
spheric air  is  also  reputed  to  be  a  mixture,  and  not  a  chemical  compound  ;  it  was  there- 
fore an  object  to  try  whether  pure  oxygen  might  not  be  obtained  by  forcing  air  through 
such  a  membrane,  filtering,  or,  in  fact,  straining  it  through  a  gum  elastic  bag.  A  thin 
piece  of  this  substance  was  therefore  tied  tightly  over  a  tube  an  inch  in  diameter  and 
six  inches  long;  the  tube  was  then  filled  with  mercury  in  such  a  manner  that  the  great 
weight  might  not  burst  the  caoutchouc  ;  it  was  then  inverted  and  exposed  to  the  atmo- 
sphere. The  membrane  bulged  into  the  tube  in  a  deep  hemispherical  form ;  in  about 
an  hour  its  under  surface  was  studded  with  bubbles  of  gas,  and  in  the  course  of  time 
several  cubic  inches  passed.  This,  on  analysis,  by  means  of  binoxide  of  nitrogen,  was 
found  not  to  differ  sensibly  from  atmospheric  air.  A  similar  result  was  also  obtained 
when  a  thin  serous  membrane,  a  piece  of  peritoneum  stripped  from  the  liver,  was  sub- 
stituted for  the  gum  elastic.  No  indications  whatever  could  be  obtained  that  atmospheric 
air  was  decomposed  during  the  process.  Nor  is  it  difficult  to  understand  and  explain 
how  this  happens,  when  a  foreign  force,  equivalent  to  a  pressure  of  six  inches  of  mer- 
cury, is  brought  to  bear  so  advantageously  on  the  action  of  a  very  thin  membrane;  for 
in  the  case  of  the  gum  elastic,  the  thickness  could  not  be  estimated  at  more  than  -jl^th. 
of  an  inch,  and  the  serous  membrane  was  so  porous  that  it  could  not  sustain  so  heavy 
a  pressure  without  innnediate  leakage;  the  united  gas,  whatever  it  may  be,  is  at  once 
forced  through,  the  barrier  being  unable  to  stop  it.  A  case  of  the  same  kind  is  met 
with  when  porous  charcoal  is  used:  pressure  forces  a  gas  through  it  entirely  unchanged; 
but  if  the  effects  of  that  pressure  be  avoided,  chemical  decompositions  of  a  decisive 
character  may  ensue,  as  we  shall  shortly  have  occasion  to  see.  To  obtain  these  chem- 
ical effects,  it  is  necessary  that  the  barrier  should  not  only  have  no  pores  of  sensible 
size,  but  that  no  adventitious  or  foreign  forces  be  brought  to  act  on  the  passing  gas ;  in 
proportion  as  these  conditions  are  fulfilled,  the  success  of  the  experiment  is  more  perfect; 
and  thus,  as  we  shall  proceed  to  point  out,  it  is  possible  to  strain  the  nitrogen  out  of 
atmospheric  air,  and  procure  by  that  means  oxygen  of  greater  or  less  purity. 


PERCOLATION  THROUGH  MASSES  OF  WATER.  j^g 

51.  The  doctrine  laid  down  in  sections  47  and  48,  of  the  condition  of  equiUbrium 
of  gases  on  each  side  of  a  membrane,  being  the  foundation  of  an  explanation  of  all  the 
phenomena  which  have  as  yet  been  noticed,  requires  farther  consideration  and  fuller 
proof  Some  remarks  have  been  offered  on  the  incomplete  results  which  are  obtained 
by  the  use  of  barriers  consisting  of  pores  of  large  size,  such  as  stucco  plugs.  It  is  said, 
however,  that  in  the  hands  of  Mr.  Graham  these  have  given  some  curious  results  re- 
specting the  rate  of  diffusion  of  gases  ;  experiments  at  once  satisfactory  and  singular. 

52.  The  objections  above  mentioned  have,  however,  appeared  to  me  so  weighty, 
that  I  have  not  made  use  of  such  barriers,  but  resorted  to  hquids,  which,  for  closeness 
of  texture,  uniformity  of  composition,  and,  above  all,  on  account  of  our  accurate  knowl- 
edge of  their  habitudes  and  structure,  are  much  preferable.  They,  also,  have  given 
results  as  curious,  but  far  more  satisfactory ;  and  though,  in  the  management  of  them, 
something  of  that  dexterity  of  manipulation  is  required  which  practice  alone  can  con- 
fer, yet  they  are  easy  of  repetition,  never  failing  to  give  precise  and  comparable  results. 
They  also  afford  the  means  of  prolonging  or  hastening  the  close  of  an  experiment,  which 
at  times  is  invaluable ;  their  action,  too,  is  very  uniform  ;  for  a  film  of  water  so  thin  as 
to  be  coloured  acts  as  well  as  a  mass  several  inches  in  depth,  but  the  gases  passing 
through  it  more  rapidly,  a  state  of  equilibrium  on  both  sides  is  obtained  in  a  few  min- 
utes. The  following  facts  will  serve  as  an  illustration:  Into  a  tube  h  {fig.  10, j)!.  1), 
which  was  conoidal  at  its  upper  end,  a  disk  of  paper,  a,  was  fastened  water-tight,  and 
then  upon  that  was  poured  distilled  water  till  it  was  about  i  inch  deep;  the  tube  was 
next  filled  at  the  pneumatic  trough  with  hydrogen  gas,  which  passed  into  the  atmosphere 
through  the  paper  roof,  and  the  water  reposing  on  it ;  but,  though  the  tube  was  only  | 
inch  in  diameter,  twenty-four  hours  elapsed  before  a  column  of  hydrogen  half  an  inch  long 
had  gone  out,  and  in  seven  days  only  one  inch  more.  A  common  glass  tumbler  was 
filled  with  hydrogen  gas  at  the  pneumatic  trough,  and  by  the  side  of  it  stood  a  small 
bottle,  the  height  of  which  was  about  1?  inch,  its  diameter  1^  inch,  and  the  diameter 
of  its  neck  f  of  an  inch.  The  atmospheric  air  in  this  bottle  being  of  the  same  tem- 
perature as  the  hydrogen  in  the  tumbler,  a  finger  dipped  in  water  rendered  slightly  viscid 
with  soap  was  passed  over  the  mouth  of  the  bottle,  so  as  to  leave  a  thin  film  stretched 
there,  the  tumbler  of  hydrogen  being  then  placed  over  it  {fig.  12,  1).  In  the  course 
of  two  minutes,  the  film,  instead  of  being  horizontal,  became  convex,  and  continued  to 
be  so  until  it  had  swelled  into  a  large  spherical  bubble,  which  capped  the  top  of  the 
bottle ;  in  sixteen  minutes  this  had  increased  so  much  in  size  and  become  so  thin  that 
it  was  of  a  dark  metaUic  lustre,  and  it  burst  at  last  by  swelling,  so  as  to  touch  the  bot- 
tom of  the  tumbler.  During  this  experiment  the  barometer  was  at  28-8  ;  thermometer 
at  68-75,  Fah. 

53.  The  rapidity  of  this  action  being  proportional  to  the  thinness  of  the  film  used 
as  a  boundary,  it  is  obvious  that  the  duration  of  an  experiment  may  be  managed  by  de- 
termining beforehand  the  thickness  of  the  film  through  which  the  gases  shall  pass.  If 
very  thick,  the  time  may  be  indefinitely  long,  and  if  very  thin,  indefinitely  short.  Nor  need 
we  be  limited  in  reducing  the  thickness  to  the  greatest  extent,  for  it  is  found  by  experi- 
ment that,  however  thin  the  film  may  be,  it  still  possesses  cohesion  enough,  and  its  parts 


20  PERCOLATION  THROUGH  EXCESSIVELY  THIN  FILMS  OF  WATER,  AS  SOAP  BUBBLES. 


are  still  so  close,  that  anything  like  mechanical  straining  or  leakage  cannot  take  place 
through  it.  The  first  attempts  to  ascertain  the  laws  of  movement  and  equilibrium  of 
gases  passing  through  liquid  films  were  made  by  stretching  those  films  over  the  mouths 
of  vials,  as  here  described ;  subsequently,  for  several  considerations,  this  arrangement 
was  given  up :  the  horizontal  film  is  at  first  too  thick,  it  exposes  too  small  a  surface  to 
the  atmosphere  to  which  it  is  subjected,  and  it  is  not  until  towards  the  close  of  the 
experiment  that  the  action  becomes  at  all  rapid.  Bubbles  of  water  made  sufficiently 
adhesive  by  a  little  soap  were,  therefore,  substituted.  One  of  these  filled  with  any  gas, 
and  immersed  in  an  atmosphere  of  another  gas,  at  once  exposes  a  large  surface,  and, 
by  swelling  or  collapsing,  allows  a  free  action.  There  are,  however,  three  circum- 
stances which  tend  to  destroy  such  bubbles,  and  against  these  provision  should  be  care- 
fully made.  Mechanical  agitations  of  the  surrounding  air  may  be  met  by  covering  the 
whole  arrangement  with  a  glass  bell.  Evaporation  from  the  surface  of  the  bubble,  which 
reduces  its  substance  unduly,  may  be  avoided  by  keeping  all  the  gases  under  trial  in 
jars  over  water,  until  they  are  loaded  with  moisture,  and  thoroughly  wetting  the  inside 
of  the  covering-bell ;  but  it  is  not  so  easy  to  prevent  that  slow  motion  of  the  parts  of 
the  bubble,  which,  in  virtue  of  the  earth's  attraction,  tends  gradually  to  bring  them  to 
the  lowest  part,  while  the  walls  of  it  become  too  thin  to  bear  the  weight,  and  are  liable 
to  burst  by  the  expansion  of  the  gases  accumulating  within. 

54.  After  a  number  of  trials,  the  following  has  been  found  to  be  the  most  suitable  ar- 
rangement for  prosecuting  these  inquiries ;  it  is  simple,  not  easily  deranged,  and  allows 
of  sufficient  latitude  and  change  to  suit  other  experiments.  In  it  a  soap  bubble  may  be 
preserved  with  certainty,  for  a  time  considerably  exceeding  an  hour,  and  sometimes  much 
longer.  As  here  described,  it  was  used  to  illustrate  the  relative  passages  of  hydrogen,  oxy- 
gen, and  nitrogen  through  a  watery  film  into  atmospheric  air.  It  is  represented  in  section; 
A  A  {Ji g.  11,  2jI'  1)  is  a  small  tin  saucer,  about  three  inches  in  diameter  and  half  an 
inch  deep ;  into  it  water  can  be  poured,  and  it  also  serves  as  a  platform  to  support  a 
large  cupping-glass,  h.  Through  the  centre  of  this  tin  saucer,  at  c,  passes  a  glass  pipe, 
f,  1  inch  in  diameter,  the  upper  extremity  of  which  is  cemented  into  a  hole  of  the  same 
size  in  a  round,  thin  piece  of  copper,  d,  which  is  about  half  an  inch  in  diameter,  the 
other  end  of  the  pipe  opening  into  another  cupping-glass,  k,  through  a  perforation  in  its 
top,  the  communication  being  capable  of  being  cut  off  by  means  of  a  cock,  g ;  the 
lower  cupping-glass  serves  as  a  support  to  the  whole  arrangement  when  placed  upon 
the  shelf  of  the  pneumatic  trough.  This  apparatus  is  used  as  follows :  The  upper 
cupping-glass  being  taken  off  the  platform,  is  filled  with  any  gas  under  trial,  as  oxygen, 
and  placed  aside  on  the  shelf  The  lower  cupping-glass  is  then  filled  with  water 
by  depressing  it  in  the  trough,  and  the  cock  being  closed,  five  hundred  measures  of 
hydrogen,  for  instance,  are  thrown  into  it.  After  seeing  that  the  copper  plate,  d,  is 
free  from  moisture,  a  drop  of  water,  rendered  slightly  viscid  by  soap,  is  placed  upon  it 
exactly  over  where  the  orifice  of  the  pipe,y^  opens.  The  upper  glass,  containing  the  ox- 
ygen, is  now  placed  upon  the  little  tin  saucer  platform,  as  in  the  figure.  The  lower 
glass  is  next  depressed  in  the  trough,  and  as  soon  as  the  cock  is  opened,  a  bubble  of 
hydrogen  containing  five  hundred  measures  expands,  the  spare  oxygen  escaping  from  the 


PERCOLATION  THROUGH  EXCESSIVELY  THIN  FILMS  OF  WATER,  AS  SOAP  BUBBLES.  21 

edge  of  the  upper  glass  through  the  water  in  the  saucer ;  the  cock  is  next  closed,  and 
the  apparatus  placed  on  the  trough  shelf  as  long  as  the  operator  desires  the  experiment 
to  continue.  Keeping  that  position  when  the  cock  is  once  more  open,  the  gas  passes 
into  the  lower  glass  until  the  bubble  is  entirely  collapsed,  when  the  cock  is  again  closed, 
the  contents  of  the  bubble  being  now  ready  for  measurement  or  analysis.  As  the  gas 
passes  from  the  bubble  into  the  lower  jar,  the  water  rises  from  the  tin  saucer  into  the 
cupping-glass  above,  confining  the  gas  that  was  outside  of  the  bubble  ;  tliis,  by  the  com- 
mon mode  of  manipulation,  is  to  be  transferred  from  the  tin  platform  to  the  shelf  of  the 
trough  for  inspection. 

55.  By  this  apparatus  it  was  found  that  one  thousand  measures  of  atmospheric  air, 
exposed  in  a  bubble  to  atmospheric  air,  in  five  successive  experiments,  underwent  no 
change  either  in  volume  or  composition.  The  duration  of  the  trials  was  severally  ten, 
fifteen,  twenty,  thirty,  and  sixty  minutes,  and  the  uniform  result,  when  drawn  back  into 
the  under  cupping-glass,  was  one  thousand  measures  exactly,  the  composition  of  which 
was  the  same  as  atmospheric  air. 

56.  The  thermometer  stood  at  54°  Fah.  One  thousand  measures  of  hydrogen  in 
the  watery  fihn  were  subjected  to  atmospheric  air  in  the  upper  bell ;  in  five  minutes 
there  remained  only  four  hundred  and  seventy-two.  In  the  second  trial,  one  thousand 
measures  in  twenty  minutes  became-four  hundred  and  thirty-two ;  and  in  a  third,  when 
the  same  quantity  of  gas  was  confined  half  an  hour,  the  residue  was  four  hundred  and 
eighty  measures. 

57.  A  reverse  action  ensues  when  nitrogen  is  substituted  for  hydrogen :  the  bubble 
swells  instead  of  diminishing,  and  the  resulting  gas  measures  more.  It  is  to  be  remarked, 
that  after  the  first  five  minutes,  provided  the  bubble  has  been  sufficiently  thin,  there 
appears  to  be  little  or  no  change  in  the  volume  of  gas,  and  in  a  great  many  experi- 
ments it  was  found  that  motion  had  ceased  when  the  bubble  had  increased  somewhere 
between  7i  and  10  per  cent.  The  thermometer  standing  at  55"  Fah.,  one  hundred  meas- 
ures of  nitrogen  in  half  an  hour  became  one  hundred  and  seven  and  a  half.  In  another 
trial,  two  hundred  measures  in  the  same  time  became  two  hundred  and  fifteen.  Again, 
two  hundred  in  fifteen  minutes  became  two  hundred  and  sixteen.  The  greatest  varia- 
tion from  this  was  in  one  case,  when,  after  an  exposure  of  five  hundred  measures  for 
five  minutes,  the  bubble  was  found  to  contain  five  hundred  and  forty-five  measures,  or 
an  increase  of  9  per  cent. 

58.  Oxygen  gas  exposed  in  hke  manner  to  atmospheric  air,  decreased  in  bulk ;  thus, 
two  hundred  and  fitty  measures  in  ten  minutes  became  one  hundred  and  fifty-three,  and 
the  like  quantity  in  fifteen  minutes  diminished  to  one  hundred  and  forty-four,  which 
amply  proves  that  the  passage  of  oxygen  takes  place  through  water  more  rapidly  than 
nitrogen.  And  upon  this  fundamental  principle,  chemical  decompositions  can  be  ef- 
fected ;  as  in  the  last  section,  where  we  have  a  bubble  of  nitrogen  gas  exposed  to  the 
atmosphere,  the  nitrogen  outside  parts  with  its  oxygen,  and  passing  through  the  barrier, 
unites  with  the  oxygen  within. 

59.  Having  thus  recognised  a  variation  in  the  rate  of  passage  of  gases  through  thin 
films,  it  becomes  a  point  of  investigation  to  ascertain  how  long  these  motions  may  be 


ANALYSES  OF  GAS  ON  THE  EXTERIOR  AND  INTERIOR  OF  SOAP  BUBBLES. 


maintained,  and  under  what  circumstances  a  state  of  equilibrium  will  ensue.  I  have 
already  stated  that  the  condition  of  rest  was  simply  an  identity  of  composition  of  the 
media  on  both  sides  the  membrane,  a  law  which  is  rigidly  observed  by  all  gases  that 
have  yet  been  tried.  Four  hundred  measures  of  nitrogen  gas  procured  by  phosphorus, 
but  which,  by  standing  over  water,  were  found  to  have  gained  3^  per  cent,  of  oxygen, 
were  exposed  to  atmospheric  air,  in  the  apparatus  above  described,  for  thirty  minutes  : 
at  the  end  of  that  time,  there  were  found  four  hundred  and  thirty-two  measures  in  the 
bubble,  of  which  15 5  per  cent,  were  oxygen.  Outside  the  bubble  were  ten  hundred  and 
seventy  measures,  which  also  contained  15|  per  cent,  of  oxygen  ;  thermometer  57"  Fah. 

60.  Two  hundred  measures  of  nitrogen,  containing  the  impurity  as  above,  were  ex- 
posed for  thirty  minutes  in  an  atmosphere  of  impure  oxygen,  which  contained  nitrogen 
and  carbonic  acid,  to  the  amount  of  13i  per  cent. :  at  the  end  of  that  time,  three  hun- 
dred and  sixty-one  and  a  fourth  measures  were  found  in  the  bubble,  of  which  62  per 
cent,  were  oxygen ;  and  eleven  hundred  and  forty-four  and  a  half  measures  were  found 
outside,  62i  per  cent,  of  which  were  oxygen ;  thermometer  55°  Fah. 

61.  Two  hundred  measures  of  oxygen  were  exposed  to  an  atmosphere  of  hydrogen 
for  fifteen  minutes,  at  a  temperature  of  66"  Fah. :  at  the  end  of  that  time,  two  hundred 
and  seven  and  three  fourths  were  found  in  the  bubble,  containing  16  f  per  cent,  of  ox- 
ygen ;  and  twelve  hundred  and  seventy-three  outside,  which  also  contained  16f  per 
cent,  of  oxygen. 

62.  The  slower  passing  gases  being  thus  found  to  obey  a  very  simple  law  of  equilib- 
rium, attempts  were  made  to  ascertain  whether  such  as  carbonic  acid,  which  are  very 
absorbable  by  water,  followed  the  same  law ;  but,  after  many  trials,  no  certain  result 
could  be  obtained,  so  rapid  was  the  action.  Five  hundred  measures  thus  confined 
passed  out  immediately,  the  bubble  collapsing  almost  as  fast  as  it  had  been  expanded  :  a 
tube  was  therefore  prepared,  which  had  a  roof  of  water  at  one  extremity,  about  half  an 
inch  thick  and  two  inches  in  diameter;  beneath  this  roof  five  thousand  measures  of  car- 
bonic acid  gas  were  placed,  and  the  arrangement  exposed  to  the  atmosphere.  In  forty- 
eigbt  hours,  analysis  showed  that  a  trace  of  carbonic  acid  still  existed  in  the  tube,  which, 
when  washed  off,  about  two  hundred  measures  of  unabsorbable  gas  remained,  con- 
sisting of  20  5  oxygen,  79-5  nitrogen ;  and  therefore  atmospheric  air.  This  experiment 
would  thus  warrant  the  conclusion  that  gases  of  any  kind  will  pass  a  barrier,  subject  to 
the  same  regulations  as  those  that  are  less  absorbable ;  had  it  been  allowed  to  continue 
for  a  sufficient  length  of  time,  there  can  be  no  doubt  that  all  the  carbonic  acid  gas  pres- 
ent would  have  escaped  into  the  atmosphere,  and  atmospheric  air  alone  been  present  on 
both  sides  of  the  barrier. 

63.  Hence,  the  condition  under  which  motion  ceases  through  a  barrier  is  identity  of 
chemical  composition  on  both  its  sides.  As  gases,  however,  pass  with  different  degrees 
of  velocity  through  the  same  liquid,  results  seemingly  anomalous  may  be  obtained,  and 
chemical  decomposition  may  ensue ;  if  water  recently  boiled  be  exposed  to  the  atmo- 
sphere, it  will  be  found  in  a  few  hours  to  have  abstracted  oxygen  and  nitrogen  gases, 
not  in  the  same  proportion,  however,  that  exists  in  the  circumambient  air,  for  the  gas 
found  in  w  ater  contains  i  instead  of  i  of  oxygen ;  perhaps  in  the  course  of  time  that 


DIFFUSION  TAKES  PLACE  BETWEEN  THE  PARTICLES  OF  HETEROGENEOUS  BODIES.  23 

richer  gas  would  escape,  and  its  place  be  taken  by  common  air.  We  therefore  con- 
sider this  a  case  in  which  equilibrium  has  not  ensued,  progress  only  being  made  to- 
wards it,  the  decomposition  and  apparent  anomaly  being  only  the  result  of  a  more  ready 
solubility  and  rapid  passage  of  one  gas.  By  taking  advantage  of  this,  it  is  possible  to 
obtain  from  the  atmosphere  oxygen  of  some  purity.  If  a  volume  of  atmospheric  air  be 
agitated  with  boiled  water  in  a  close  vessel,  it  will  be  found  that  a  rapid  absorption  of 
its  oxygen  ensues,  while  but  little  nitrogen  is  imprisoned  among  the  pores  of  the  liquid. 
This  gas,  by  the  action  of  heat,  may  be  driven  off  from  the  water,  and  being  subjected 
to  another  washing,  may  be  rendered  still  more  pure ;  by  successively  washing  and 
rejecting  the  nitrogen  left,  a  gas  so  rich  in  oxygen  may  be  procured  as  to  be  equal  to 
some  that  is  obtained  by  other  processes,  as  by  the  action  of  sulphuric  acid  on  perox- 
ide of  manganese. 


CHAPTER  IV. 

ON  INTERSTITIAL  MOVEMENTS,  heitig  «  Continuation  of  the  preceding  Chapter. 

{From  the  American  Journal  of  the  Medical  Sciences  for  May,  1836.) 

Contents  :  Diffusion  takes  place  heticeen  the  Particles  of  Heterogeneous  Bodies. — Dif- 
fers from  Chemical  Attraction. — Action  of  Dinary  Arrangements. — Action  of  Ternary 
Arrangements. — Decompositions  hy  Binary  Arrangements. — Decompositions  hy  Ter- 
nary Arrangements. 

64.  If  we  could  place  a  known  volume  of  vapour  in  the  centre  of  an  extensive  void, 
where  no  disturbance  from  without  could  solicit  its  particles  to  move  in  one  direction 
rather  than  another,  it  is  to  be  supposed  that,  conformably  to  certain  laws  that  are 
known  to  obtain  and  operate  on  bodies  of  an  aerial  constitution,  movement  would  en- 
sue. To  an  assignable  limit  the  vapour  would  expand,  by  a  species  of  repulsion  of  its 
own  particles.  In  the  immense  vacuum  in  which  the  solar  system  moves,  there  are  orbs 
that  seem  to  fulhl  this  condition  ;  these,  though  they  wander  through  very  large 
paths,  and  are  disturbed  by  the  reaction  of  bodies  they  move  past,  sufficiently  approxi- 
mate the  circumstances  here  laid  down  to  show  that  there  is  an  extent  beyond  which 
bodies  so  constituted  are  not  disposed  to  expand.  Astronomical  observations  also  show 
that  gases  such  as  our  own  atmosphere  is  composed  of  do  not  by  their  expansion  tres- 
pass beyond  a  given  point  into  a  void,  for  then  the  laws  upon  which  they  are  formed 
react,  as  the  firmest  barrier  from  without  would  do,  to  prevent  their  farther  expansion. 

65.  An  orb  so  constituted  could  not,  in  any  length  of  time,  undergo  any  change  of  com- 
position, structure,  or  figure  ;  for,  so  soon  as  the  first  motion  which  decided  its  equilibrium 
was  over,  that  equilibrium  would  remain  undisturbed,  unless  forces  from  without  were 
brought  to  bear  upon  it.  In  a  void  such  as  we  are  here  supposing,  apart  from  any  such 
derangement,  equally  a  void  as  to  force  as  well  as  to  matter,  the  vaporous  mass  could 
not  be  subject  to  any  contingency. 


24 


DIFFERS  FROM  CHEMICAL  AFFINITY. 


66.  Let  US  extend  our  supposition  by  placing  another  volume  in  presence  of  the  for- 
mer, differing  from  it  in  chemical  composition  alone.  That  difference  would  determine 
certain  motions  of  penetration,  in  addition  to  those  resulting  from  mere  mechanical  action. 
Not  only  would  the  united  mass  move  so  as  to  assume  a  mechanical  equilibrium,  but 
its  constituent  parts  would  also  move,  so  as  to  establish  a  chemical  equilibrium  at  the  same 
time.  Wherever  an  atom  of  one  of  the  vapours  existed,  there  would  be  found  one  of 
the  other  also.  To  bring  about  this  result,  a  mutual  penetration  of  parts  is  demanded,  a 
transit  of  the  constituents  of  one  vapour  among  those  of  the  other.  The  motions  tbat 
effect  this  arrangement  take  place  without  any  resistance,  just  in  the  way  that  the  hglit 
of  a  distant  star  comes  into  our  system,  undeterred  by  the  rays  of  our  sun,  and  moves 
freely  in  every  direction ;  his  beams  also  move  in  the  vacuum,  intersecting  the  paths  of 
other  luminous  bodies  without  any  hinderance  or  shock. 

67.  In  this  state  of  extension,  when  the  component  atoms  of  a  gas  or  vapour  are  sup- 
posed to  be  stretched  to  their  utmost  limit,  which  we  are  prone  to  imagine  can  only  be 
done  by  an  increase  of  the  distance  usually  existing  between  an  atom  and  its  neigh- 
bours, it  is  not  difficult  to  suppose  that  these  different  motions  can  go  on,  and  that  a 
foreign  atom  may  insinuate  itself  in  the  interstices  between  others.  But  our  ideas  of 
space  and  size  being  only  relative,  and  as  we  know  nothing  of  the  dimensions  of  an  ulti- 
mate atom,  nor  of  the  interval  that  parts  it  from  those  around  it,  it  is  plain  we  could  not, 
without  actual  experiment,  determine  when  a  body  had  arrived  at  that  state  of  con- 
densation, or  when  its  particles  had  become  so  closely  approximated  to  each  other  as  to 
refuse  the  admission  of  foreign  atoms  between  them. 

68.  A  mass  of  any  kind  in  a  vacuum,  and  undisturbed,  moves,  therefore,  only  in  that 
manner  which  the  laws  of  dynamics  indicate.  Motions  of  another  kind,  however,  are 
induced  when  the  vacuum  is  changed  for  a  substance  ;  a  kind  of  penetration,  permeation, 
or  absorption  is  the  result ;  nor  do  the  mechanical  conditions  of  bodies  appear  to  have 
any  effect :  with  some  of  these  phenomena  we  are  familiar.  A  gas,  a  liquid,  or  a  solid 
may  indiscriminately  pass  by  solution  into  the  pores  of  water  without  any  reference 
to  their  aggregation.  A  variety  of  words  have  been  used  to  express  this  action  :  solution, 
endosmosis,  permeation,  &c. ;  but,  parting  from  the  simplest  experimental  condition,  we 
shall  have  occasion  to  see  that  all  these  refer  to  varieties  of  one  phenomenon  only. 

69.  If  a  solitary  body  has  thus  no  opportunity  of  exhibiting  the  conditions  of  its 
own  arrangement  as  to  structure  or  the  forces  that  inhabit  the  interstices  of  its  atoms,  it  is 
very  different  with  a  binary  arrangement.  Chemists  are  familiar  with  the  phenomena 
exhibited  when  gases,  solids,  or  liquids  are  exposed  to  each  other  under  those  circum- 
stances where  no  direct  change  of  composition  ensues.  Thus,  if  a  cubic  inch  of  carbonic 
acid  gas  be  exposed  to  a  cubic  inch  of  water,  the  gas  in  a  short  time  passes  into  the 
Uquid  mass,  or  is  absorbed  by  it,  with  a  certain  degree  of  force  and  to  a  certain  amount. 
Also,  aqueous  gas  rises  from  the  water,  and  diffuses  itself  into  the  un absorbed  remainder 
of  the  carbonic  acid.  After  a  sufficient  time,  no  part  of  the  carbonic  acid  will  be  found 
destitute  of  aqueous  gas,  nor  will  any  part  of  the  water  be  without  its  equivalent  of 
carbonic  acid.  The  simplest  example  of  these  combinations  is  furnished  by  the  solution 
of  saline  bodies  in  water,  where  there  is  no  change  of  chemical  composition,  but  merely 


DIFFERS  FROM  CHEMICAL  AFFINITY.  25 

a  detachment  of  the  sohd  crjstalhne  particles  from  the  mass  of  the  dissolving  substance; 
these  pass  among  the  interstices  of  the  liquid,  and  remain  there,  unaffected  bj  gravity, 
being  equally  and  uniformly  diffused.  Of  the  powers  by  vs  hich  this  is  brought  about  we 
are  not  well  informed,  but  no  fact  in  science  is  better  ascertained  than  this  uniform  and 
equable  diffusion.  If,  by  affinity,  we  mean  a  power  that  causes  substances  to  unite  with 
an  interchange  of  elements,  or  one  which  is  only  exerted  to  bring  about  an  alteration  of 
composition,  such  a  force  is  obviously  insufficient  to  give  rise  to  these  effects. 

70.  That  one  particle  has  the  power  of  attaching  itself  to  another  of  a  dissimilar 
kind,  without  anything  like  change  of  composition,  numerous  facts  demonstrate.  The 
delicate  dyes  that  adhere  to  cloth-fibre  offer  an  example  ;  they  cannot  be  supposed  to  be 
attached  by  any  force  affecting  either  their  composition  or  structure,  since  the  successful 
operations  of  the  artist  proceed  upon  the  supposition  that  the  tint  shall  be  unimpaired, 
and  the  strength  and  organization  of  the  fibre  which  is  dyed  shall  remain  untouched. 
Now  in  those  cases  where  we  know  that  the  dyeing  material  acts  chemically  on  the 
fibre,  is  there  not  abundant  proof  that  the  elementary  changes  affect  the  uniting  bodies  ? 
Is  not  the  hue  of  the  dye  changed,  and  does  not  the  fabric  become  rotten  1  Other 
facts  also  show  that  these  adhesions,  without  chemical  change,  are  possible ;  the  foil 
on  the  back  of  a  mirror  is  not  retained  by  the  exercise  of  any  force  which  has  brought 
about  a  change  in  its  composition.  When  the  dye  is  washed  off,  or  the  foil  scraped 
away,  the  cloth-fibre  and  the  looking-glass  are  both  found  in  their  original  integrity  of 
structure. 

71.  The  cases  here  cited  furnish  examples  ot  one  solid  uniting  to  another  in  a  man- 
ner that  involves  something  different  from  the  action  of  chemical  affinity.  There 
is  a  whole  range  or  class  of  similar  combinations  :  a  solid  may  unite  thus  with  a  liquid, 
as  sugar  and  water  ;  a  liquid  with  a  liquid,  as  alcohol  and  water ;  a  liquid  with  a  gas,  as 
carbonic  acid  and  water ;  or  a  gas  with  a  gas,  as  oxygen  and  nitrogen.  All  these  are 
cases  where  there  is  no  interchange  of  chemical  elements,  and  which  we  cannot,  there- 
fore, suppose  to  ensue  in  virtue  of  chemical  force. 

72.  Although  these  actions  are  the  result  of  a  kind  of  adhesion  of  particle  to  particle, 
and  might,  therefore,  be  supposed  to  take  place  in  an  indiscriminate  or  irregular  man- 
ner, there  are  some  remarkable  circumstances  attending  them  which  go  to  show  the 
contrary ;  thus,  water  will  dissolve  a  certain  quantity  of  sulphuric  ether,  and  no  more  ; 
it  will  take  up  its  own  volume  of  carbonic  acid,  and  no  more ;  it  will  hold  in  solution 
of  bisulphate  of  potash,  sulphate  of  ammonia,  protosulphate  of  iron,  bicarbonate  of 
potash,  chromate  of  potash,  muriate  of  strontian,  &c.,  half  its  weight,  at  60  F.  At  the 
same  temperature  it  dissolves  its  own  weight  of  sulphate  of  magnesia,  and  this  com- 
parison might  be  extended  much  farther.  The  same  kind  of  predilection  for  definite 
quantities  obtains  also  in  gases,  as  is  the  case  with  atmospheric  air,  where  the  propor- 
tions of  oxygen  dissolved  in  nitrogen  are  as  one  to  four,  nearly. 

73.  All  these  things  go  to  prove  that  the  passage  of  the  particles  of  one  body  among 
the  particles  of  another  proceeds  upon  certain  and  definite  laws.  Whether  the  resi- 
dence of  sahne  atoms  among  the  interstices  of  a  liquid  is  a  phenomenon  of  the  same 
sort  as  the  adherence  of  dye  to  a  fibre,  it  is  not  material  to  inquire.    We  know,  by 

D 


26  ACTION  OF  BINARY  ARRANGEMENTS. 

experiment,  that  a  solitary  gas  has  a  tendency  to  expand  itself  to  a  certain  extent,  but 
not  farther;  and  we  are  equally  assured  that  bodies,  whether  of  the  same  or  of  dif- 
ferent kinds,  have  an  inclination  to  penetrate  into  each  other.  Where  there  is  an  ap- 
parent indisposition  to  do  this,  we  are  not  without  plausible  reasons  for  supposing  it  to 
be  through  the  intervention  of  disturbing  causes.  If  oil  and  water  do  not  commingle,  it 
is  a  result  determined  by  the  action  of  their  cohesion,  as  compared  with  the  force  of 
attraction  between  them.  An  interesting  example  of  this  nature  is  afforded  by  the  ac- 
tion of  mercury  on  glass  :  under  ordinary  circumstances,  they  show  no  disposition  to 
unite,  not  even  so  much  as  water  and  oil ;  but,  by  a  suitable  application  of  heat,  the  co- 
hesion of  the  mercury  may  be  so  lessened,  and  its  force  of  attraction  for  glass  at  the 
same  time  so  exalted,  that  it  can  be  brought  to  icet  it ;  an  experiment  first  successfully 
performed  by  Laplace. 

74.  This  nisus,  or  endeavour  of  one  body  to  diffuse  itself  into  the  interstices  of  an- 
other, has,  under  a  variety  of  forms,  been  long  recognised.  The  solution  of  salts,  the 
absorption  of  gas  by  liquids,  the  passage  of  liquids  through  crystals,  the  permeation  of 
porous  textures,  the  diffusion  of  gases,  the  languid  movement  occurring  in  solids,  were 
known  long  ago.  Of  late  years,  some  extension  of  these  facts  has  been  obtained,  and 
the  new  phenomenon,  though  explicable  on  the  same  principles,  is  dignified  by  the  title 
ENDOSMOSIS  (37). 

75.  For  the  explanation  of  the  whole  of  this  most  interesting  series  of  results,  one 
postulate  alone  is  demanded — that  all  bodies  have  a  tendency  to  diffuse  themselves  into 
the  interstices  of  all  others,  with  more  or  less  intensity.  Nor  is  it  difficult  to  admit  this 
principle  in  its  fullest  extent,  when  we  consider  the  numerous  examples  philosophy  af- 
fords of  it.  All  kinds  of  chemical  absorptions  and  solutions  are  cases  of  it.  The  dis- 
turbing causes  which  sometimes  change,  or  even  entirely  hinder  these  actions,  we  shall 
consider  hereafter. 

76.  Binary  arrangements,  or  those  in  which  two  bodies  are  engaged,  whether  solid, 
liquid,  or  gaseous,  exhibit  some  circumstances  which  it  is  here  necessary  to  point  out. 
Let  us  suppose  the  couple  under  consideration  to  be  oxygen  gas  exposed  to  an  equal 
volume  of  water.  No  remarkable  phenomena  attend  the  passage  of  the  gas  into  the 
liquid  ;  there  is  no  rise  of  temperature,  and  the  whole  amount  absorbed  is  greatly  less 
than  the  bulk  of  the  water.  If  another  gas  be  substituted,  as  carbonic  acid,  though 
much  more  soluble,  there  is  still  no  indication  of  change  of  temperature,  but  ammonia 
and  muriatic  acid  condensing  to  a  much  greater  amount,  disengage  heat.  Another 
couple  might  be  assumed,  as  charcoal  or  porous  masses,  with  oxygen  or  other  gases,  and 
similar  indications  be  obtained. 

77.  If,  after  a  liquid  has  absorbed  as  much  of  any  given  gas  as  it  is  capable,  we  re- 
move the  remnant  of  unabsorbed  gas,  and  in  its  place  substitute  some  other  of  a  differ- 
ent kind,  complex  reaction  ensues.  The  gas,  already  absorbed  by  the  water,  has  its 
condition  of  equilibrium  disturbed,  and,  in  conformity  with  the  general  principle  (75),  it 
has  a  tendency  to  diffuse  itself  out  of  the  water  into  the  newly-introduced  gas.  This, 
in  its  turn,  has  also  a  tendency  to  pass  into  the  water.  Thus,  if  over  a  volume  of  water, 
impregnated  with  carbonic  acid,  and  confined  in  a  jar  over  mercury,  we  place  a  vol- 


ACTION  OF  TERNARY  ARRANGEMENTS.  27 

lime  of  oxygen,  equilibrium  would  not  be  obtained  until  a  certain  amount  of  carbonic 
acid  was  found  in  the  gas,  and  a  certain  amount  of  oxygen  in  the  water.  And  the 
same  would  hold  in  the  case  of  any  other  gases  or  any  other  liquid.  In  the  course  of 
experiment,  examples  of  this  case  are  often  met  with.  The  water  commonly  used  in 
pneumatic  troughs  contains  both  oxygen  and  nitrogen.  If  into  a  jar  containing  such 
water  we  pass  pure  nitrogen,  in  the  course  of  a  few  minutes  oxygen  will  leave  the  wa- 
ter to  diffuse  itself  into  the  nitrogen.  Had  we  thrown  in  pure  oxygen,  nitrogen,  on  the 
contrary,  would  have  deserted  the  water  and  mingled  with  the  oxygen  gas.  In  gaseous 
analysis,  this  action,  which  obtains  to  a  greater  or  less  extent  with  every  gas,  often  gives 
rise  to  much  perplexity. 

78.  Ternary  Arrangements. — It  is  plain  that  the  conditions  of  the  action  consid- 
ered in  the  last  paragraph  may  be  obtained  at  once  by  suitable  arrangements  ;  and,  as 
it  is  important  that  these  should  be  well  understood,  I  shall  dwell  upon  them  minutely. 

79.  In  paragraph  (77),  we  considered  the  reaction  ensuing,  first,  of  a  single  couple 
or  binary  arrangement,  and  then  the  disturbance  effected  by  the  introduction  of  another 
element.  Could  we,  then,  at  once  have  exposed  the  volume  of  water  by  one  surface  to 
oxygen  gas,  and  by  another  to  carbonic  acid,  the  changes  that  were  consecutive  would 
have  been  simultaneous.  Let  a  {Jig.  13,  pi.  1)  be  a  sheet  of  water,  on  which,  at  its 
upper  surface,  a  volume,  h,  of  carbonic  acid  reposes,  and  beneath  its  under  surface,  c,  a 
volume  of  oxygen  ;  both  gases  pass  at  once  through  the  water,  in  opposite  directions,  into 
each  other.  It  is  evident  that  the  thinner  we  make  the  barrier  of  water,  the  more  rap- 
idly will  equilibrium  be  obtained.  This  I  have  accomplished  in  the  following  manner, 
by  using  mere  liquid  films,  and  for  that  purpose  have  taken  advantage  of  soap  and  other 
bubbles.  A  glass  tube  {  inch  in  the  bore,  and  seven  or  eigiit  inches  long,  is  to  be  drawn 
out  at  one  extremity  to  a  capillary  termination,  and  when  the  bubble  is  to  be  blown,  the 
other  end  is  dipped  into  a  solution  of  soap.  The  tube  having  been  previously  passed 
through  a  cork,  as  in  jig.  14,  p)l.  1,  is  now  to  be  introduced  into  a  clear  vial  or  bell 
glass,  the  neck  of  which  the  cork  fits  loosely ;  on  blowing  at  the  capillary  termination, 
the  bubble  slowly  expands  in  the  vial,  where  it  is  protected  from  access  of  air.  To 
measure  its  diameter,  I  take  a  strip  of  white  pasteboard,  and  divide  it  into  inches  and 
decimals,  placing  it  in  such  a  position  before  the  vial  that  it  crosses  the  bubble  diamet- 
rically; then  with  a  small  telescope  that  magnifies  twelve  or  twenty  times,  and  at  the 
distance  of  about  eight  feet,  I  observe  the  bubble  much  magnified,  the  micrometrical 
pasteboard  apparently  passing  through  its  very  substance,  as  is  shown  in  the  figure. 

80.  Through  a  soap  bubble  1-53  inch  in  diameter,  the  substance  of  which,  previous 
to  expansion,  was  contained  in  a  cylinder  i  inch  in  diameter  and  \  in  height,  ammonia, 
either  pure  or  diluted  with  atmospheric  air,  passes  instantaneously  when  air  from  the 
lungs  is  on  the  other  side.  Into  the  bottle  in  which  the  bubble  is  to  be  blown  a  little 
strong  solution  of  ammonia  is  to  be  poured  ;  the  bubble  is  then  expanded  ;  at  a  particular 
point  it  becomes  dyed  with  the  richest  hues,  and  that  moment  the  phenomenon  of 
endosmosis  is  complete :  care  must  be  had  to  suffer  no  moisture  from  the  mouth  to 
close  the  capillary  termination  of  the  glass  tube ;  and  now  a  rod,  a,  dipped  in  muriatic 
acid,  is  to  be  brought  over  the  opening ;  as  the  bubble  is  collapsing  by  the  attraction  of 


28 


ACTION  OF  TERNARY  ARRANGEMENTS. 


its  own  parts,  dense  fumes  of  muriate  of  ammonia  make  their  appearance,  which  continue 
until  the  substance  of  the  bubble  has  entirely  returned  into  the  tube.  The  extraordinary 
rapidity  of  this  action  is  remarkable.  The  bubble  is  scarcely  blown  before  it  is  full  of 
auunonia ;  and  it  is  not  less  interesting  to  observe  how  the  colours  play  with  change 
of  atmosphere.  A  little  cylinder  expanded  to  the  size  of  a  pea,  which,  in  common 
air,  is  opaque  white,  and  which  would  not  be  coloured  until  expanded  to  six  or  eight 
times  that  diameter,  becomes  deeply  tinged  as  soon  as  it  is  penetrated  by  ammonia. 
If  restored  to  the  free  atmosphere  it  loses  all  its  beauty,  and  these  alterations  may  be 
kept  up  at  pleasure  by  merely  changing  it  from  one  medium  to  another. 

81.  When,  for  the  purpose  of  experiment,  it  is  desirable  to  have  a  permanent  bub- 
ble, a  small  column  of  moisture  from  the  tongue  must  be  allowed  to  close  the  capillary 
termination  of  the  tube. 

82.  In  the  same  manner,  hydrosulphate  of  ammonia  is  found  to  pass  with  instanta- 
neous rapidity  through  the  film,  and  may  be  detected  by  a  paper  dipped  in  acetate 
of  lead.  The  colours  in  this  case  become  very  quickly  stable,  as  with  ammonia,  and 
do  not  produce  that  iridescent  play  which  the  passage  of  certain  other  substances 
affords.  It  is,  however,  essential  to  the  success  of  these  experiments  that  the  sub- 
stances about  to  be  passed  through  the  film  shall  not  have  any  chemical  action  upon  it. 
Thus,  it  is  not  possible  to  use  muriatic  acid,  which  decomposes  soap,  but  there  is  no 
difficulty  in  the  management  of  such  as  oxygen,  hydrogen,  nitrogen,  &c.  The  passage 
of  hydrogen  through  these  films  is  exemplified  in  the  following  table  : 

Diameter  of  a  bubble  of  hydrogen  gas  exposed  to  atmospheric  air.    Diameter  of  a  bubble  of  atmospheric  air  exposed  to  hydrogen  gas. 


lixp. 

When  Blown. 

In  two  Minutes. 

* 

1 

460 

430 

430 

2 

•415 

■390 

390 

3 

•425 

■400 

■400 

Exp. 

When  Blown. 

In  two  Minutes. 

* 

1 

•470 

■485 

"495 

2 

■360 

■375 

380 

3 

•420 

■435 

•445 

83.  The  third  column,  marked  *,  in  these  tables  was  taken  when  the  black  spot  on 
the  top  of  the  bubble  was  about  half  an  inch  in  diameter ;  for,  as  the  coloured  rings 
were  the  same  in  each  experiment,  and  the  surface  incapable  of  reflecting  light  of  equal 
extent,  it  is  to  be  presumed  that  the  measures  were  obtained  under  like  circumstances, 
as  far  as  the  thickness  of  the  film  was  concerned.  In  all  cases  the  bubbles  were  blown 
by  pressure  on  a  gum  elastic  bag.  This  method  of  measuring  the  expansion,  though 
suitable  for  general  purposes,  cannot,  however,  be  extensively  relied  on,  owing  to  thermal 
disturbance  and  the  earth's  action  changing  the  figure  from  a  true  sphere  to  a  prolate 
spheroid. 

84.  It  is  interesting  to  remark  with  what  extraordinary  rapidity  these  permeations 
take  place.  If  we  expand  a  small  bubble  in  a  vessel  of  ammonia,  hydrogen,  sulphuretted 
hydrogen,  &c.,  by  means  of  the  mouth,  and,  without  removing  the  lips  from  the  capillary 
opening  of  the  tube,  inhale  immediately  the  contents  of  the  bubble,  the  gaseous  matter 
will  impress  the  organs  of  taste  with  a  very  distinct  savour,  peculiar  to  the  gas  on  which 
the  experiment  is  tried.  There  is  a  class  of  vapours  which  appears  to  possess  little 
or  no  affinity  for  water,  such  as  ether  and  the  essential  oils ;  these,  however,  percolate 
through  tissues  of  water  with  rapidity.   On  covering  the  bottom  of  a  vial  with  oil  of  pep- 


I 


ACTION  OF  TERNARY  ARRANGEMENTS.  29 

permint,  aud  then  expanding  a  bubble,  the  taste  of  the  essential  oil  will  be  perceived 
when  a  portion  of  the  air  is  drawn  back  out  of  the  bubble  into  the  mouth.  With  other 
oils,  as  cajeput,  and  with  ethers,  the  effect  is  the  same ;  and  it  is  to  be  observed,  that 
during  the  transit  they  work  the  surface  of  the  bubble  into  a  kind  of  microscopic  waves, 
and  produce  an  iridescent  play  of  colours. 

85.  To  obviate  any  exception  that  might  be  taken  to  the  use  of  soapy  matter  in  these 
films,  or  to  their  excessive  thinness,  I  have  employed  the  arrangement  of  section  52, 
which  establishes  the  same  truths. 

86.  Into  such  a  tube  I  threw  200  meastires  of  hydrogen  gas,  and  the  same  quantity 
into  one  the  upper  extremity  of  which  was  hermetically  sealed,  by  way  of  affording  a 
comparison.  In  the  former,  the  thickness  of  the  roof  of  water  was  about  i  inch,  and  in 
24  hours  the  level  of  the  water  in  it  rose  half  an  inch  ;  while  in  the  latter  it  remained  un- 
disturbed, thus  incontestably  proving  that  hydrogen  gas  passes  with  great  freedom  through 
masses  of  water.  Nor  is  this  permeability  confined  to  that  liquid  alone :  a  tube  which 
was  thus  covered  with  a  layer  of  lamp  oil,  in  five  days  raised  the  level  of  the  water  in 
it  more  than  two  inches  ;  and  one  the  roof  of  which  was  of  copaiba  balsam,  threw  out 
all  the  gas  to  within  {  of  an  inch  of  its  top ;  while  a  tube  of  the  same  size,  but  sealed 
at  the  other  end,  that  stood  by  them,  kept  its  level. 

87.  It  appears  to  me  the  reason  that  we  have  not  hitherto  understood  the  phenom- 
ena of  endosmosis,  or  the  action  of  these  ternary  arrangements,  as  I  have  called  them, 
has  arisen  chiefly  from  the  employment  of  substances  as  barriers,  which  were  possessed 
of  pores  of  sensible  size.  A  moment's  consideration  will  place  this  in  its  true  light : 
suppose  two  gases  were  kept  apart  by  the  intervention  of  a  plug  of  charcoal,  in  their 
diffusion  into  each  other,  not  only  would  those  portions  pass  the  barrier  which  were 
brought  along  by  a  direct  action,  but  a  much  larger  quantity  would  slip  through  by 
mere  leakage  among  the  pores  (36).  Bladder,  tissues,  stucco  plugs,  &c.,  which  we 
know  to  possess  pores  of  sensible  size,  are  open  to  this  objection  ;  but  the  case  is  very 
different  with  liquids,  which,  from  their  uniform  condition  and  the  close  proximity  of 
their  atoms,  admit  of  no  such  action.  A  mass  of  stucco  a  foot  thick  would  be  subject 
to  this  kind  of  mechanical  derangement,  but  a  sheet  of  water  reduced  to  that  excessive 
degree  of  thinness  that  it  is  invisible,  allows  no  gas  to  go  through  it  by  leakage,  but  all 
passes  by  absorption. 

88.  The  original  experiment  of  Dutrochet  on  endosmosis,  and  those  of  Dr.  Mitchell, 
were  examples  of  this  class  of  ternary  arrangements.  In  these  cases,  membranes  or  gum 
elastic  were  tied  over  the  mouths  of  vessels,  and  the  result  was  shown  by  the  swelling 
or  sinking  of  the  barrier.  These  can  be  repeated  in  a  more  satisfactory  manner  with 
liquids  (52). 

89.  In  this  experiment  we  also  recognise  an  identity  of  results  with  those  which  have 
heretofore  excited  so  much  attention,  under  the  title  Endosmose  ;  but  understanding  in 
this  case,  as  we  do,  the  conditions  under  which  the  result  is  obtained,  there  is  no  dif- 
ficulty in  extending  the  explanation  of  one  experiment  to  the  other.  Endosmose  is  only 
a  complex  case  of  simple  absorption.  The  mechanical  results  here  obtained,  the  swell- 
ing or  sinking  of  the  barrier,  depend  on  the  more  rapid  absorption  of  one  gas  by  that 


30 


DECOMPOSITIONS  BY  BINARY  ARRANGEMENTS. 


barrier.  The  condition  under  which  we  obtain  the  mechanical  resuk  will,  by  being 
duly  varied,  also  furnish  chemical  results,  an  investigation  of  which  forms  our  next  object. 

90.  Che3iical  Decomposition;  «»^/,^r.s^,  %  binary  arrangements. — A  solitary  ar- 
rangement of  any  kind,  whether  it  be  of  a  simple  or  of  a  compound  nature,  has  no  pow- 
er of  change  in  itself ;  but  it  is  conceivable  that  one  of  the  latter  kind — compound — on 
forming  part  of  a  binary  arrangement,  may  be  differently  affected ;  as  an  illustration,  let 
us  take  atmospheric  air  and  water,  appropriately  situated,  to  form  such  an  arrangement 
(63).  In  this  case,  were  the  nitrogen  and  oxygen  equally  absorbable  by  the  liquid,  no 
remarkable  result  would  ensue ;  but  such  is  not  the  fact ;  the  oxygen  gas  passes  much 
more  quickly  into  the  water  than  the  nitrogen,  and  decomposition  takes  place,  an  ex- 
cess of  oxygen  being  in  the  liquid,  and  an  excess  of  nitrogen  being  left.  We  should, 
therefore,  expect  that  rain,  and  dew,  and  springs,  and  rivers,  which  have  been  exposed 
in  a  very  divided  state  to  the  air,  ought  to  contain  a  gas  richer  in  oxygen  than  that  of 
the  atmosphere ;  and  such,  in  fact,  is  the  case,  the  atmosphere  containing  one  volume 
of  Oxygen  and  four  of  nitrogen,  the  gas  of  water  containing  one  of  oxygen  and  two  of 
nitrogen,  as  we  shall  shortly  find. 

91.  Instead  of  a  gas  and  a  liquid  to  form  these  binary  arrangements,  a  solid  and  a 
gas  may  be  used.  Into  500  measures  of  atmospheric  air,  a  piece  of  charcoal,  that  had 
been  made  red  hot  and  quenched  under  mercury,  was  placed.  The  volume  of  the  air 
experienced  a  rapid  diminution,  and  after  the  absorption  had  gone  on  for  several  hours, 
there  remained  205  measures,  100  of  which  contained  only  eight  of  oxygen.  The 
charcoal  was  now  introduced  into  water  over  mercury,  and  commenced  very  actively 
evolving  gas,  which  contained  only  3-75  per  cent,  of  oxygen,  and  the  last  portions  of  it 
that  were  given  off,  only  2*8.  Solution  of  lime  was  not  capable  of  detecting  the  pres- 
ence of  carbonic  acid  in  the  water. 

92.  In  the  place  of  charcoal,  other  porous  solids  might  be  substituted ;  into  a  jar,  a, 
which  contained  atmospheric  air,  there  was  introduced  a  piece  of  red-hot  pumice  stone; 
into  &,  a  piece  of  clay  that  had  been  made  red  hot ;  and  into  c,  a  piece  of  charcoal 
quenched  under  water.  Absorption  took  place  in  them  all,  and  in  a  quarter  of  an  hour 
a  was  found  to  contain  19  per  cent,  of  oxygen,  shortly  after  6  was  found  to  contain  19 
per  cent,  of  oxygen,  and  c,  in  half  an  hour,  only  18  per  cent.  Also,  in  five  hours,  c  only 
contained  17-25  of  oxygen,  and  in  seven  days,  only  seven  per  cent.;  but,  at  the  same 
time,  a  and  h  contained  14*50  per  cent.    Four  days  after,  c  contained  only  five  per  cent. 

93.  By  long  boiling,  I  extricated  all  the  air  possible  by  such  a  process,  from  a  quan- 
tity of  water,  and  pouring  it  into  a  glass  cup,  left  it  exposed  to  the  atmosphere  for  some 
days;  at  the  end  of  that  period  the  water  was  again  boiled  in  a  close  vessel,  and  the 
gaseous  matter  it  had  absorbed  submitted  to  analysis.  After  the  carbonic  acid  had  been 
carefully  washed  off,  its  amount  being  about  29  per  cent.,  it  was  found  that  the  residue 
contained  32 1  per  cent,  of  oxygen  gas.  It  is  a  singular  fact,  that  an  aqueous  mass,  in 
thus  decomposing  atmospheric  air,  appears  to  follow  a  very  simple  law;  pure  spring  water 
and  distilled  water,  after  a  competent  exposure  to  the  atmosphere,  are  found  to  contain 
a  gas  whose  elements  are  not  in  the  proportion  of  one  to  four,  as  in  the  case  with  the 
atmosphere,  but  in  the  proportion  of  one  to  two.    In  several  analyses  of  the  air,  extrica- 


DECOMPOSITIONS  BY  TERNARY  ARRANGEMENTS. 


31 


ted  by  boiling  from  tbe  water  of  a  spring,  which  flows  from  a  sandy  valley,  and  also 
from  the  dews  which  fall  on  a  neighbouring  hill,  but  too  remote  to  be  affected  by  the 
exhalations  of  dwellings,  I  found  the  proportion,  when  care  was  taken  in  the  analysis, 
to  be  uniformly  33^  per  cent.,  or  as  1  to  2  by  volume.  This  gas,  thus  extricated,  is 
isomeric  with  protoxide  of  nitrogen,  with  the  particular  exception  that,  in  the  protoxide, 
the  two  volumes  of  nitrogen  are  compressed  into  half  their  bulk, 

94.  In  a  quart  jar,  which  was  filled  with  spring  water  and  inverted  into  a  tin  cap- 
sule, I  collected  all  the  aeriform  matter  that  could  be  disengaged  from  the  water  by 
means  of  a  fire  placed  beneath  the  jar  and  its  tin.  This  gas,  from  many  prior  trials,  I 
knew  to  contain  33, V  per  cent,  of  oxygen.  When  all  the  gas  was  collected  that  could 
be  extracted,  at  a  temperature  long  continued  close  upon  the  boiling  point,  the  arrange- 
ment was  suffered  to  cool,  and  kept  undisturbed  for  four  days ;  at  the  close  of  that  time, 
considerably  more  than  three  fourths  of  the  gas  disengaged  was  reabsorbed;  the  residue, 
on  analysis,  contained  5*25  per  cent,  of  oxygen  only.  A  portion  of  the  water  in  the 
jar  was  now  submitted  to  a  boiling  temperature  in  a  small  close  vessel,  and  the  gas  col- 
lected was  analyzed.  It  contained,  instead  of  33^,  rather  more  than  47  per  cent,  of 
oxygen.  There  cannot,  therefore,  be  any  doubt  that  oxygen  may  be  obtained  from  the 
atmosphere,  in  a  pure  and  undiluted  state,  by  the  action  of  a  tissue,  or  a  binary,  and 
also  a  ternary  arrangement. 

95.  Decompositions  by  Ternary  Arrangements. — After  this  consideration  of  the 
case,  in  which  two  elements  are  employed,  we  are  prepared  to  understand  how  ternary 
arrangements  effect  decompositions.  Let  h  {fig.  1^,  pi.  1)  be  a  compound  gas,  which 
is  placed  above  a  barrier,  a,  of  such  a  nature  that  one  of  the  elements  of  h  shall  pass 
more  rapidly  through  it,  or,  in  other  words,  be  more  readily  absorbed  by  it  than  the 
other.  Also,  let  the  other  substance,  c,  which  is  on  the  opposite  side  of  the  barrier,  be 
of  a  kind  capable  of  removing  the  quicker  passing  element  of  h  from  the  under  surface 
of  a,  as  fast  as  it  arrives  there.  It  is  immaterial  how  this  removal  be  accomplished, 
whether  by  chemically  uniting  with  it,  or  by  mechanical  action ;  the  quick  passing  el- 
ement, finding  at  its  approach  to  the  under  surface  of  the  barrier  a  ready  exit,  continu- 
ally passes  off,  and  its  place  is  supplied  by  fresh  portions  from  above,  so  that,  in  the 
lapse  of  time,  only  the  less  absorbable  element  will  be  found  in  h. 

96.  The  general  conditions,  therefore,  of  chemical  decomposition  by  ternary  arrange- 
ments are,  that  one  element  of  the  compound  to  be  decomposed  shall  pass  more 
easily  through  the  barrier  or  bounding  tissue  than  the  others,  and,  on  its  arrival  at  the 
opposite  side  of  the  barrier,  it  shall  be  rapidly  removed. 

97.  Reasoning  upon  this  principle,  I  succeeded,  nearly  two  years  ago,  in  effecting 
decompositions  in  this  manner,  which  have  some  important  physiological  applications. 
Having  taken  a  tube,  one  of  the  ends  of  which  was  expanded  into  a  trumpet-shape,  and 
closed  with  a  thin  serous  membrane — peritoneum  stripped  from  the  liver — which  was 
tightly  tied  on  with  a  waxed  thread,  while  it  was  wet,  I  poured  through  the  orifice, 
which  was  open,  a  strong  but  clear  solution  of  litmus  in  wateiv  The  tube  thus  situated 
was  placed  in  a  wine-glass  containing;  strong  alcohol,  and  the  level  of  the  liquid,  inside 
and  outside,  made  to  coincide.    The  conditions  for  decomposition  were  thus  fulfill- 


32 


DECOMPOSITIONS  BY  TERNARY  ARRANGEMENTS. 


ed ;  the  water  could  find  a  ready  passage  through  the  serous  membrane,  but  the  col- 
ouring matter  could  not.  Now,  on  arriving  at  the  under  side  of  the  membrane,  the 
water  either  was  removed  by  uniting  chemically  with  the  alcohol,  or  by  sinking  me- 
chanically through  it  to  the  bottom  of  the  glass.  Complete  decomposition  was  effected, 
all  the  colouring  matter  being  retained  above  the  membrane,  and,  on  placing  a  candle 
on  one  side  of  the  glass  and  the  eye  on  the  other,  dense  striae  of  colourless  water  were 
seen  passing  through  the  alcohol,  but  not  a  particle  of  the  litmus  escaped. 

98.  Under  this  condition,  those  experiments  which  have  been  instituted  to  demon- 
strate the  passage  of  colouring  matter  through  the  lacteals  have  been  made.  The  lac- 
teals  do  not  open  into  an  intestine  with  patulous  mouths,  but  their  lining  membrane  of 
serous  tissue  ends  bluntly  in  a  kind  of  cul-de-sac.  Through  such  a  membrane,  litmus, 
indigo,  &c.,  cannot  penetrate,  though  water  may  find  a  ready  passage.  Hence,  be- 
cause we  cannot  colour  the  chyle  by  an  injection  of  litmus  or  indigo  water,  it  is  not  to 
be  inferred  that  no  medicine  can  pass  from  an  intestine  into  the  lymphatic  system ;  the 
experiment  now  detailed  goes  to  prove  directly  the  reverse,  and  furnishes  us  with  an 
explanation  of  the  uniformity  of  colour  of  the  fluid  in  the  chyliferous  vessels. 

99.  An  important  circumstance  in  gaseous  analysis  may  here  be  noticed.  If  a  tissue, 
in  the  act  of  transmitting  gas,  or  ready  to  do  so,  be  placed  in  contact  with  another  gas 
of  a  different  nature,  disturbance  immediately  ensues.  A  cubic  inch  of  nitrogen  gas, 
made  with  phosphorus,  but  which  was  found  to  be  contaminated  with  4|  per  cent,  of 
oxygen,  was  agitated  briskly  in  a  vial  containing  about  an  ounce  of  spring  water,  such 
as  has  been  mentioned  to  contain  a  gas  ^  oxygen.  In  one  minute  the  nitrogen  gained 
one  per  cent,  by  the  agitation.  The  same  quantity  of  nitrogen  gas,  agitated  in  a  pint 
of  water,  gained  no  less  than  11  per  cent,  of  oxygen,  which  it  had  taken  from  the  rich 
gas  of  the  water.  Nor  is  agitation  or  mechanical  violence  necessary  to  produce  this 
important  result.  Into  a  bell  filled  with  water,  and  inverted  into  another  vessel,  so  as 
not  to  touch  it  in  any  point,  I  threw  100  measures  of  a  gas,  85  of  which  were  oxygen. 
After  four  weeks  an  analysis  was  made,  and  the  gas  in  the  bell  found  to  contain  only  72 
per  cent,  of  oxygen,  the  remainder  being  nitrogen.  In  this  way,  too,  in  the  lapse  of 
time,  from  an  inverted  vessel,  partially  filled  with  atmospheric  air,  the  oxygen  will  es- 
cape into  the  water,  and  thence  into  the  atmosphere ;  and  I  have  twice  known  this 
event  to  take  place,  so  that  the  residue  did  not  contain  more  than  three  or  four  per 
cent,  of  oxygen.  In  many  of  the  most  delicate  researches  of  chemistry,  we  have  this 
disturbing  cause  in  operation.  Water  is  universally  employed  in  our  laboratories  as  a 
means  of  confining  gases ;  it  enters  largely  into  our  processes  of  pneumatic  manipula- 
tion ;  and  though  we  have  hitherto  neglected  its  action,  it  silently  disturbs  all  our  re- 
sults. An  air-bell  cannot  pass  to  the  top  of  a  jar  without  instant  contamination  :  du- 
ring its  residence  there  it  is  subject  to  a  continued  succession  of  changes — at  no  two 
moments  is  it  the  same  in  composition — a  perfect  freedom  of  communication  existing 
between  it  and  the  atmosphere. 

100.  As  an  instrument  of  rigid  analysis,  the  pneumatic  apparatus,  so  arranged,  requires 
to  be  used  with  circumspection.  It  is  impossible  to  keep  oxygen,  nitrogen,  or  any  other 
gas  in  its  original  purity,  if  confined  by  water.    This  fluid,  which,  when  reduced  to  a 


DECOMPOSITIONS  BY  TERNARY  ARRANGEMENTS. 


33 


thin,  imperceptible  film,  is  instantaneously  permeated  by  almost  every  substance,  under- 
goes the  hke  action  in  course  of  time,  even  in  deep  masses.  Gases  are  absorbed  by  it, 
and  thrown  off  by  it  in  its  purest  state ;  how  much  more  complicated,  then,  must  its 
action  be  in  that  impure  condition  in  which  it  is  commonly  used.  Connected  with  this 
point  there  is  another  :  if  a  series  of  bells  stand  on  a  pneumatic  trough,  each  will  affect 
all  the  others,  communicating  a  part  of  its  contents,  and  receiving  from  them  in  return. 
A  jar,  containing  binoxide  of  nitrogen,  standing  by  the  side  of  one  containing  com- 
mon air,  seriously  affects  it.  I  have  noticed  two  common  tumblers,  filled  with  these 
gases,  and  so  placed,  communicate  with  each  other  so  freely,  that  in  17  hours  the  tum- 
bler originally  filled  with  atmospheric  air  contained  only  9f  per  cent,  of  oxygen.  The 
habit  of  collecting  gases  at  the  same  trough,  that  is  destined  to  preserve  others,  is  very 
exceptionable  ;  we  place  the  disturbing  agency  in  circumstances  the  most  favourable 
for  its  action.    All  operations  of  washing  are  hable  to  the  same  strictures. 

101.  We  have  assumed  it  as  a  law  of  nature,  that  any  substance,  when  placed  in  con- 
tact with  another,  has  a  tendency  to  diffuse  into  it. 

102.  It  is  to  be  remarked  in  reference  to  this,  that  no  hypothetical  cause  is  assumed; 
it  is  merely  taken  as  one  of  those  ultimate  facts  which  the  progress  of  knowledge  has 
not  explained.  We  do  not  consider  whether  it  involves  the  position  that  two  bodies 
can  exist  in  one  place  at  one  time,  nor  do  we  deny  the  impenetrability  of  matter. 
But  it  is  required  of  us  by  a  crowd  of  facts  to  admit  this  law,  as  the  only  legitimate 
position  on  which  they  can  be  explained.  We  know  nothing  of  the  size,  or  figure,  or 
condition  of  the  ultimate  atoms  of  bodies;  there  are,  indeed,  some  circumstances  which 
would  lead  us  to  suppose  that,  even  in  the  densest  structures,  each  particle  is  at  an 
immense  distance  from  those  that  are  next  around  it,  in  comparison  with  its  own 
diameter.  In  those  interstices  which  must  necessarily  exist,  these  phenomena  of  ab- 
sorption may  take  place  in  accordance  with  laws  which  obtain  among  the  molecules 
of  bodies.  In  the  same  way  that  a  comet  comes  down  from  the  regions  of  space  and 
traverses  a  planetary  system,  receiving  impressions  greater  or  less  from  each  star  that 
it  passes,  and  emerges  back  again  untouched  and  unimpaired ;  so  a  gaseous  particle 
may  pass  through  the  system  of  atoms  that  constitute  a  solid  mass,  and  moving  therein 
unimpeded  and  without  contact  with  any  of  them,  may  emerge  without  change  of 
physical  condition,  or  only  a  mark  that  its  motion  has  been  subject  to  those  laws  which 
obtain  in  the  system  through  which  it  has  gone. 

103.  All  these  observations  go  to  establish  the  point,  that  pores  of  a  sensible  size 
have  nothing  to  do  with  endosmosis — that  it  is  a  phenomenon  depending  simply  on 
absorption.  No  one  would  aver  that  water  possessed  any  apertures,  or  vessels,  or  tubu- 
lar arrangement, 

104.  The  experiment  of  (80)  does  not  alone  prove  that  endosmosis  takes  place 
through  liquids  and  tissues  the  pores  of  which  have  no  sensible  size;  it  has  a  much  more 
interesting  application.  Physiologists  know  that  the  primitive  form  of  all  organic  bod- 
ies is  an  imperforate  vesicle  or  globule,  having  the  power  of  absorbing  those  substances 
which  are  around  it,  and  decomposing  them.  The  ultimate  vesicle  yields  to  analysis 
carbon,  the  elements  of  water,  and  a  few  salts.    It  is  a  centre  of  vital  activity,  a 

E 


34 


IMPORTANCE  OF  CAPILLARY  ATTRACTION  IN  PHYSIOLOGY. 


laboratory  assimilating  things  for  its  own  substance.  The  simplest  plants,  confervas, 
tremellae,  and  the  simplest  animals,  consist  alone  of  this  structure.  Let  us  observe 
how  nearly  this  vesicle  agrees,  both  in  its  constitution  and  mode  of  action,  with  the 
vesicle  of  (80).  Like  that,  it  is  not  only  an  imperforate  cell,  but  also  consists  of  simi- 
lar elements.  The  properties  which  the  organized  vesicle  is  supposed  to  enjoy  are 
met  with  in  the  fullest  extent  in  that  which  is  not  organized.  Both  have  powers  of 
endosmosis,  and  a  species  of  assimilation  of  things  exterior  to  their  own  substance. 
What  property  has  the  lowest  order  of  animal  and  vegetable  life  which  that  bubble 
does  not  possess  t  A  thing  thus  endowed  with  vitality  may  well  excite  our  interest ; 
it  breathes,  it  is  nourished,  it  exhales. 

105.  By  referring  the  phenomenon  of  endosmosis  to  absorption,  such  as  has  been 
recognised  by  chemists,  we  advance  one  point  in  the  simpUfication  of  our  knowledge. 
It  gives  us  also  a  better  idea  of  the  specific  action  of  tissues,  as  depending  on  structural 
arrangement,  and  presents  an  intricate  problem  in  its  easiest  form  for  solution;  more- 
over, it  is,  as  I  know  by  experience,  a  safe  guide  in  experimental  research.  We  can 
hardly  doubt  that  the  forces  bringing  about  the  result  indicated  in  (80)  are  the  same  as 
those  which  operate  in  Dr.  Mitchell's  experiment,  where  Lidia-rubber  is  used  as  a  bar- 
rier ;  and  if  that  result  receives  so  ample  and  so  easy  an  explanation  upon  this  doc- 
trine, why  should  we  hesitate  to  apply  it  to  the  other?  But  the  composition,  structure, 
and  habitudes  of  a  thin,  watery  film,  are  much  better  known  than  those  of  a  lamina  of 
India-rubber :  we  can  reason  with  certainty  respecting  the  one,  and  vary  its  composi- 
tion to  suit  the  purposes  of  experiment ;  the  other  aifords  no  such  advantage.  If,  how- 
ever, it  should  eventually  be  found  that  the  simple  doctrine  of  absorption  is  not  suffi- 
cient to  explain  all  the  phenomena  of  endosmosis  that  may  hereafter  be  discovered,  this 
paper  will  at  least  prove  that  the  cause  of  those  phenomena  is  not  alone  enjoyed  by 
organic  and  solid  tissues,  but  also  by  liquids  and  substances  witJwut  organization. 


CHAPTER  V. 

THE   PHYSICAL  THEORY  OP   CAPILLARY  ATTRACTION. 

{From  the  American  Journal  of  the  Medical  Sciences  for  February,  1838.) 

Contents  :  Importance  of  Capillary  Attraction  in  Fhysiology. — Capillary  Attraction 
is  an  Electrical  Phenomenon. — Its  Physical  Theory. —  The  Effect  varies  loith  Varia- 
tions of  Electric  Disturbance. —  Takes  place  between  Bodies  of  different  Forrns. — 
Physiological  Illustrations. 

106.  It  has  been  alleged,  as  a  bar  to  all  physiological  investigation,  that  the  phenom- 
ena of  life  are  of  so  peculiar  a  nature,  that  we  must  necessarily  forever  remain  ignorant 
of  their  causes  ;  that,  unlike  physical  phenomena,  which  are  of  a  simpler  kind,  and  more 
\vithin  the  reach  of  human  understanding,  there  is  something  in  these  inherently  mysie- 


IMPORTANCE  OF  CAPILLARY  ATTRACTION  IN  PHYSIOLOGY. 


35 


rious  and  incomprehensible.  This  unphilosophical  impression  exists  not  only  in  the 
minds  of  the  vulgar,  but  has  extended  itself  to  men  well  trained  to  scientific  research ; 
it  is  to  be  found  in  the  writings  of  the  most  eminent  physicians,  and  often  affords  a 
plausible  screen  for  professional  ignorance.  Of  all  the  sciences,  medicine  is  the  last  to 
profit  by  the  analytic  method — a  method  which  has  raised  other  departments  of  knowl- 
edge to  their  present  rank.  Its  cultivators  pursue  the  same  course  of  synthesis  which 
was  pursued  in  the  days  of  the  Greeks — they  reason  from  hypothesis  to  fact,  instead 
of  from  fact  to  hypothesis. 

107.  It  may,  however,  be  boldly  declared,  that  the  science  of  life  is  not  more  occult 
than  any  other  of  the  sciences.  We  may,  by  proper  investigation,  carry  it  as  far ;  and 
we  shall  only  stop  short  at  the  very  same  point  which  has  proved  impassable  in  them. 
Of  final  causes  we  know  nothing;  the  immediate  agent  of  life  is  not  more  obscure 
than  any  of  the  remote  physical  agents.  If  we  cannot  assign  any  reason  why  a  seed 
germinates,  can  we  tell  why  a  stone  falls  to  the  earth  1  is  the  one  phenomenon  any  more 
comprehensible  than  the  other  1  If  we  cannot  tell  how  it  is  that  one  parent  should 
produce  a  countless  offspring,  each  of  which  has  the  power  of  reproducing  beings  like 
itself,  neither  can  we  tell  how  a  spark  produces  an  extensive  conflagration.  It  avails 
us  little  to  say  that  the  principle  of  life,  like  the  prijiciple  of  heat,  possesses  a  radiant 
character,  or  has  a  power  of  self-production.  We  are  equally  ignorant  how  the  wide- 
spreading  flame  results  from  a  spark,  and  how  countless  myriads  of  seeds  have  origina- 
ted from  one  primordial  germ. 

108.  Some  parts  of  the  science  of  physiology  are  doubtless  within  the  reach  of  scien- 
tific investigation.  Most  of  the  functions  of  organic  life  are  of  this  character.  Absorp- 
tion, secretion,  circulation,  and  respiration  are  carried  on  through  the  medium  of  tubu- 
lar arrangements  of  different  kinds,  endued  with  specific  powers.  We  are  not  well  in- 
formed of  the  nature  of  these  actions,  nor  of  the  force  giving  rise  to  them.  The  changes 
taking  place  in  organic  structures  partake  partly  of  a  mechanical,  partly  of  a  chemical 
aspect,  bearing  some  similarity  to  other  physical  changes  effected  by  known  agents,  yet 
not  identical  with  them.  Some  have  supposed  that  the  attraction  of  affinity,  or  the  force 
of  capillarity,  was  the  power  in  question,  operating  in  an  unusual  manner,  under  unusual 
circumstances  ;  but  the  majority  of  medical  writers  have  cut  the  knot,  instead  of  untying 
it,  and  assert  that  it  is  a  peculiar  force,  recognised  under  the  title  of  vital  force,  life,  or 
nature. 

109.  It  is,  however,  most  unphilosophical  to  resort  to  these  vain  explanations,  which, 
after  all,  afford  us  no  information,  substituting  only  obscure  terms  as  the  causes  of  events 
not  more  obscure.  Had  we  approached  the  problem  of  pore-action  in  the  same  spirit 
that  has  led  to  the  development  of  the  causes  of  magnetic  action,  a  similar  and  equally 
striking  advance  would  have  been  made. 

110.  Capillary  attraction,  considered  simply  as  a  mechanical  force,  is  not  competent 
to  produce  those  changes  which  the  pores  and  narrow  cylinders  of  organic  structures 
give  rise  to.  The  products  of  glandular  action  are  chiefly  compounds  of  a  definite 
number  of  equivalents,  bearing  a  strong  resemblance  to  the  products  of  ordinary  chem- 
ical action ;  but  still  the  operation  of  capillarity  as  a  force  producing  motion  is  undeni- 


36 


IT  IS  AN  ELECTRICAL  PHENOMENON. 


able.  Can  it  also  produce  chemical  changes  1  Is  it  simply  a  manifestation  of  the  elec- 
tro-chemical relations  of  matter  ? 

111.  Previous  to  entering  at  large  into  an  examination  of  the  laws  of  pore-action, 
this  query  will  demand  an  answer.  We  shall  find,  from  what  follows,  that  capillary  at- 
traction is  a  force  nearly  allied  to,  if  not  identical  with,  chemical  affinity.  Now  the  in- 
vestigation of  the  problem  of  pore-action  naturally  divides  itself  into  two  parts.  1st. 
The  mechanical  conditions  of  equilibrium  and  movement  of  fluids  residing  in  tubes  of 
narrow  diameter,  but  of  any  length.  2d.  The  chemical  changes  which  fluids  so  situ- 
ated undergo. 

112.  The  identification,  therefore,  of  the  force  producing  the  mechanical  effect, 
with  that  producing  the  chemical  changes,  is  a  most  important  point,  and  to  this  I  shall 
direct  my  attention. 

113.  There  are  two  phenomena  of  capillary  attraction,  the  conditions  and  circum- 
stances of  which  are  well  known  :  the  rise  and  depression  of  fluids  in  tubes  of  a  cer- 
tain diameter,  and  the  adhesion  of  flat  solid  plates  to  the  surface  of  fluids.  From  the 
former  of  these  this  kind  of  attraction  has  derived  its  name;  the  latter  furnishes  us  with 
the  means  of  making  researches,  devoid  of  ambiguity,  in  reference  to  the  physical  cause 
of  capillarity. 

114.  If  a  circular  disk  of  glass,  or  any  other  solid  substance  {fig.  lb,  pi.  1),  a  b,  be 
placed  on  the  surface  of  any  fluid,  ef,  by  means  of  a  handle  c,  it  will  adhere  thereto 
with  a  certain  force,  which  may  be  measured  by  means  of  a  balance,  but  which  is  suf- 
ficiently evident  when  attempts  are  made  to  lift  the  disk  with  the  hand.  This  force  is 
known  under  the  name  of  capillary  attraction.  An  investigation  of  its  physical  cause, 
and  the  laws  respecting  it,  involves  the  fundamental  propositions  of  pore-action  and 
passage  through  tissues. 

115.  The  phenomena  of  capillarity  are  brought  about  by  electricity  operating  under 
peculiar  circumstances.  They  are  due  to  a  disturbance  of  the  electric  equilibrium,  and 
hence  are  intimately  allied  to  all  kinds  of  chemical  and  vital  changes. 

116.  Let  a  b  {fig.  IG)  be  a  glass  plane  reposing  on  the  surface  of  mercury,  c  d,  con- 
tained in  an  insulated  vessel,  and  capable  of  being  elevated  by  an  insulating  handle,  e  ; 
let  the  mercury  be  connected  with  an  electrometer,  /j  by  means  of  a  wire.  Now,  so 
long  as  the  glass  plane  and  the  mercury  are  in  contact,  the  electrometer  evinces  no  dis- 
turbance :  but  as  soon  as  the  plane  «  &  is  raised  by  its  insulating  handle,  electricity  is 
instantly  developed,  and  the  gold  leaves  diverge.  As  there  was  no  electrical  excitement 
while  the  plane  and  the  metal  were  in  contact,  it  is  a  legitimate  inference  that  the  elec- 
tricity now  developed  was  the  cause  of  their  strong  attraction  or  adhesion ;  and  this  is 
corroborated  on  taking  the  glass  plane  to  another  electroscope,  when  it  will  be  found 
that  it  is  electrified  positively  and  the  mercury  negatively ;  and  that,  consequently,  when 
they  are  brought  into  the  vicinity  of  each  other,  a  powerful  attraction  must  result. 

117.  A  cause  of  attraction  being  thus  developed,  it  would  be  very  unphilosophical 
to  seek  for  other  agencies  where  one  so  competent  to  produce  all  the  eflccts  is  observed 
to  exist.  For  in  every  case  where  a  solid  plane  reposes  on  the  surface  of  a  fluid  not 
wetting  it,  a  large  amount  of  electricity  of  very  high  tension  is  produced,  the  electricity 


ITS  PHYSICAL  THEORY. 


37 


of  the  surface  of  the  plane  being  always  opposite  to  that  of  the  liquid.  They  must, 
therefore,  attract  each  other,  I  express  here  only  a  fact,  not  involving  any  disputed  hy- 
pothesis whatever,  as  to  whether  that  development  of  electricity  originates  in  the  mere 
contact  of  ilie  bodies,  their  chemical  action,  or  any  other  cause ;  but  it  is  a  fact,  that 
when  any  solid  reposes  upon  any  fluid,  provided  its  surface  does  not  become  wetted,  a 
development  of  electricity  uniformly  takes  place,  and  a  powerful  degree  of  attraction 
must  necessarily  ensue. 

lis.  The  postulate  here  introduced  requires  explanation,  for  electric  excitement  is  not 
observed  if  the  solid  surface  is  wetted.  Solids  bear  a  peculiar  relation  to  liquids,  being 
wetted  or  not  wetted  by  them.  Most  solids,  for  instance,  are  wetted  by  water,  and  but 
few  by  mercury ;  the  surface  of  the  glass  is  readily  moistened  by  alcohol  or  oil,  but  not 
by  melted  sulphur  or  mercury ;  hence  the  latter,  from  its  not  adhering  to  the  skin,  was 
called  by  the  older  chemists  aqua  non  madefociens  manus.  The  circumstance  that  no 
electrical  excitement  is  observed  when  a  solid  surface  is  wet,  might  appear,  at  first  sight, 
conti'adictory  to  the  hypothesis  here  assumed.  A  more  accurate  examination,  however, 
places  it  in  a  very  different  light,  and  shows  that  the  phenomena  observed  are  exactly 
such  as  they  ought  to  be  hypothetically.  If  a  disk  of  glass  is  placed  on  the  surface  of  the 
water  and  then  removed,  the  gold  leaves  of  the  annexed  electroscope  are  not  affected,  for, 
strictly  speaking,  no  rupture  has  taken  place  between  the  solid  and  the  fluid ;  the  thin 
film  of  the  latter  in  contact  with  the  former  still  remains  so  :  it  is  only  the  cohesion  of 
the  watery  particles  that  is  overcome,  not  the  adhesion  of  the  solid  to  the  fluid,  and 
hence  no  electrical  development  appears. 

119.  Geometers  have  shown  the  exact  relation  a  solid  must  bear  to  a  fluid  to  be  wet- 
ted by  it.  It  results  from  the  mathematical  investigations  of  Clairaut,  that  if  the  at- 
traction of  the  particles  of  the  solid  for  those  of  the  fluid  is  more  than  half  the  attrac- 
tion of  these  last  for  each  other,  the  solid  will  be  wetted ;  but  if  it  be  less  than  half,  the 
solid  will  not  be  wetted.  An  experimental  proof  of  this  may  be  obtained  by  counter- 
poising a  disk  of  glass,  at  the  end  of  one  of  the  arms  of  a  balance,  by  weights  in  the 
scale,  and  then  lowering  it  on  the  surface  of  some  mercury  in  a  cup ;  it  will  be  found 
that  a  certain  weight  must  be  added  in  the  scale  to  detach  it.  Next,  in  place  of  the 
disk  of  glass,  substitute  a  plate  of  amalgamated  copper,  of  the  same  size  and  weighty 
and  ascertain  the  force  required  to  detach  it ;  this  wiU  uniformly  be  found  more  than 
double  the  former  weight.  The  first  weight  expressed  the  attractive  force  existing  be- 
tween a  surface  of  glass  and  mercury  ;  the  second,  the  cohesion  of  a  cylinder  of  mer- 
cury of  the  same  diameter,  and  the  numbers  obtained  experimentally  corroborate  the 
investigations  of  Clairaut. 

120.  I  dwell  on  this  part  of  the  phenomenon  because  it  is  of  no  smaU  importance ; 
the  same  conditions  that  determine  whether  or  not  the  surface  of  a  solid  is  to  be 
wetted,  determine  also  whether  a  liquid  shall  pass  through  a  pore,  and  move  forward 
in  a  capillary  vessel. 

121.  The  difficulty  arising  from  the  non-development  of  electricity,  where  the  solid 
surface  is  wetted,  being  thus  dismissed,  we  next  inquire  whether  the  hypothesis  here 
assumed  will  give  numerical  results  analogous  to  those  procured  by  experiment ;  in 


38 


THE  EFFECT  VARIES  WITH  VARIATION  OF  ELECTRIC  DISTURBANCE. 


Other  words,  if  two  solids  which  adhere  to  a  certain  fluid,  with  forces  differing  in 
amount,  develop  upon  rupture  quantities  of  electricity  in  the  same  ratio.  As  a  general 
result,  the  balance  and  electrometer  prove  that  this  is  the  case.  Bees'  wax,  which  ad- 
heres to  mercury  with  much  less  force  than  gum  lac,  develops  likewise  much  less 
electricity.  Gum  lac,  which  adheres  less  strongly  than  glass,  likewise  develops  much 
less  electricity ;  but  when  we  attempt  to  run  a  comparison  in  this  manner  among  a 
series  of  substances,  we  find  there  are  many  disturbing  causes  which,  in  most  cases,  in- 
capacitate us  entirely  from  making  comparable  results.  Much  depends  on  the  relative 
conducting  power  of  the  surface  employed.  A  plate  of  iron  may  be  separated  from  a 
surface  of  mercury,  which  does  not  wet  it,  with  a  very  small  disturbance  of  electric 
equilibrium,  arising  from  the  high  conducting  power  of  the  metallic  plate,  which  enables 
a  transfer  of  any  free  electricity  to  take  place  if  the  plate  should  tilt  on  one  side,  or  any- 
thing affect  its  horizontality  during  the  act  of  separation.  In  proportion  as  the  con- 
ducting power  increases,  although  the  force  of  adhesion  may  remain  the  same,  the  total 
effect  on  the  electrometer  should  diminish  ;  and  this  is  agreeable  to  experience.  Again, 
the  presence  of  moisture  on  any  part  of  the  touching  surfaces  will  vitiate  the  results ; 
partly  owing  to  its  high  conducting  power,  but  chiefly  to  the  circumstance  that  it  hin- 
ders the  surfaces  under  trial  from  ever  coming  into  contact. 

122.  The  circumstance  of  this  great  variability  in  the  amount  of  developed  electricity 
is  in  itself  strong  evidence  of  relationship  between  the  supposed  cause  and  the  effect. 
Gay-Lussac  found  that  it  required  a  weight,  sometimes  of  158,  and  sometimes  of  296 
grammes,  to  detach  a  certain  disk  of  glass  from  mercury,  depending  on  causes  which 
were  not  very  apparent.  An  effect  thus  differing  in  amount  indicates  a  cause  of  like 
variability,  or  subject  to  many  disturbances. 

123.  I  regard,  therefore,  the  agent  bringing  about  capillary  phenomena  as  identical 
with  that  producing  chemical  action,  and  both  as  being  due  to  electricity.  The  force 
of  cohesion  bears  the  same  relation  to  both,  acting  on  both  as  a  disturbing  power. 
Nay,  we  may  even  take  a  much  more  extensive  view  of  the  matter,  and  from  the  ratio 
these  forces  bear  to  each  other  predicate  the  effect  of  their  combined  action,  which  may 
be  classed  under  three  distinct  heads. 

1st.  If  the  force  of  attraction  of  the  particles  of  a  solid  for  the  particles  of  a  fluid  be 
not  equal  to  half  the  cohesive  force  of  the  latter  for  each  other,  the  fluid  will  refuse  to 
pass  through  a  pore  of  that  solid  substance,  and  in  capillary  vessels  consisting  of  it  will 
be  depressed  below  its  hydrostatic  level. 

2d.  If  the  force  of  electric  attraction  of  the  particles  of  a  sohd  for  the  particles  of  a 
fluid  exceeds  half  the  cohesive  force  of  the  latter  for  each  other,  but  is  not  equal  to  the 
whole  force,  the  fluid  will  pass  through  a  pore  formed  of  that  solid  substance,  and  in  a 
capillary  tube  of  it,  will  rise  above  its  hydrostatic  level. 

3d.  If  the  force  of  electric  attraction  of  the  particles  of  a  solid  for  the  particles  of  a 
fluid  exceed  the  whole  cohesion  of  the  latter,  chemical  union  ensues. 

124.  In  thus  assimilating  the  force  producing  pressure  on  planes  and  motion  in  nar- 
row pipes,  with  the  force  producing  chemical  changes  in  the  constitution  of  bodies,  a 
great  advantage  is  gained  in  simplifying  physiological  investigations  in  respect  of  the 


I 


TAKES  PLACE  BETWEEN  BODIES  OF  DIFFERENT  FORMS.  39 

action  of  capillary  systems.  It  is  an  electrical  force  that  determines  all  kinds  of  con- 
stitutional changes  developed  in  bodies  by  the  chemistry  of  organic  life,  and  it  is  a 
manifestation  of  the  very  same  force  that  carries  some  fluids  along  the  almost  invisible 
vessels  of  living  structures,  and  denies  to  others  a  passage.  All  the  phenomena  of  inor- 
ganic chemistry  are  the  result  of  the  balancings  of  the  force  of  cohesion  on  the  one 
hand,  and  electrical  attraction  on  the  other.  If  BerthoUet  was  wrong  in  supposing 
that  chemical  affinity,  as  an  acting  force,  had  no  existence,  other  chemists  have  equally 
erred  in  supposing  that  all  kinds  of  changes,  without  any  limitation,  were  due  to 
it.  Whether  we  investigate  the  phenomena  of  chemistry  or  of  capillarity,  we  have 
the  same  forces  to  deal  with,  acting  as  antagonists  to  each  other ;  and  hence  the  whole 
effects  imputed  to  capillary  attraction  may  be  regarded  as  belonging  to  that  extensive 
class  which  the  science  of  chemistry  considers. 

125.  There  is  a  variety  of  facts  recorded  by  writers  on  capillary  attraction,  which  an 
application  of  these  principles  readily  explains,  though  hitherto  they  have  been  regarded 
by  philosophers  as  remarkable  anomalies.  Such  is  the  observation  of  Huygens,  that 
it  was  possible  to  cause  mercury  to  stand  in  a  barometer  seventy  inches  high;  or  that 
of  P.  Abat,  of  a  singular  deviation  in  the  hydrostatic  level  of  the  same  fluid  in  different 
branches  of  a  siphon. 

126.  Capillary  attraction  does  not  take  place  only  between  solids  and  fluids ;  it  is 
exhibited  when  solids  alone  are  made  use  of.  In  virtue  of  this  power,  two  pieces  of 
lead  cohere  with  great  energy  to  each  other,  as  also  is  the  case  with  two  planes  of  pol- 
ished stone  or  plates  of  glass.  When  glass  is  used,  electricity  of  very  high  tension  is 
readily  detected,  one  of  the  pieces  being  positive  and  the  other  negative ;  it  would,  I 
suppose,  hardly  be  denied  that  the  force  operating  in  the  case  of  glass  is  also  the  force 
that  operates  in  the  case  of  stones.  Is  it  not,  then,  a  legitimate  supposition,  that  the 
adhesion  of  two  pieces  of  lead  is  brought  about  by  the  same  agent,  the  presence  of 
which  is  masked  by  the  high  conducting  power  of  the  metal  1 

127.  Between  solids  and  gases  capillary  action  likewise  takes  place.  On  the  sur- 
face of  all  kinds  of  solids  atmospheric  air  remains  in  a  state  of  condensation,  as  is  made 
evident  when  such  bodies  are  placed  beneath  water  under  an  exhausted  receiver  ;  the 
air  appearing  in  copious  bubbles,  studding  the  surface  of  the  metal. 

128.  Now  having  a  power  the  operation  of  which  over  inorganic  masses  is  so  exten- 
sive, it  is  for  us  to  inquire  how  far  the  phenomena  of  organic  systems  depend  upon 
its  working.  Those  numerous  pores,  and  pipes,  and  capillary  vessels  which  abound  in 
all  kinds  of  living  structures,  but  of  the  action  of  which  we  are  so  ignorant,  point  out  to  us 
capillary  attraction  as  one  of  the  great  forces  in  play,  determining  all  kinds  of  motions 
and  physical  changes.  To  identify  the  force  producing  motion  of  a  mechanical  char- 
acter with  that  effecting  physical  change,  gives  a  unity  to  the  action  of  powers  which 
have  hitherto  been  multiplied  without  avail,  and  stamps  simplicity  and  symmetry  on 
actions  that  are  very  diverse. 

129.  It  is  not  alone  in  the  vital  functions  that  we  meet  with  applications  of  the  prin- 
ciples of  capillary  action;  the  mechanical  functions  furnish  numerous  instances.  The 
organs  of  progression  of  some  animals  which  delight  to  walk  upon  water  are  provided 


40 


PHYSIOLOGICAL  ILLUSTRATIONS. 


with  an  apparatus  of  hair,  calculated  to  repel  that  fluid ;  hence  gnats  and  certain  other 
insects  have  no  difficulty  in  passing  over  the  surface  of  water.  By  the  same  means  the 
hydra  suspends  itself,  without  effort,  in  that  element ;  for  having  exposed  for  a  time  the 
extremity  of  its  foot  to  the  air,  so  that  it  may  hecome  dry,  it,  by  repulsion,  forms  a  cup- 
shaped  hollow  around  it,  the  head  of  the  insect  hanging  down  in  the  water  beneath. 

130.  Organs  of  exhalation  and  absorption  are  unquestionably  capillary  systems. 
The  stomata  of  plants,  which  botanists  suppose  to  discharge  these  functions,  are  of  this 
character ;  they  furnish  a  well-marked  instance  of  the  accommodation  of  apparatus  to 
suit  physical  conditions.  Plants  growing  beneath  the  surface  of  water  have  no  stomata  ; 
but  if,  by  any  means,  they  reach  the  atmosphere  and  vegetate  in  it,  these  organs  are  pro- 
duced for  the  purpose  of  discharging,  under  the  new  order  of  things,  offices  which  were 
accomplished  by  other  means.  The  spongioles  of  roots,  acting  as  capillary  systems, 
drive  the  fluids  they  absorb  from  the  earth,  through  the  tubular  vessels  of  trees,  with  a 
force  of  several  atmospheres,  extending  themselves  at  a  due  distance  from  the  trunk, 
where  they  may  meet  with  the  water  that  falls  from  the  leaves.  In  some  orders  of 
living  things,  which  are  not  accommodated  with  distinct  orifices  for  the  reception  of 
food,  nutrition  is  accomplished  by  capillary  systems.  In  this  manner  the  porifera  ex- 
pose a  wide  surface  to  the  seas,  and  draw  in  nutrient  matter  through  their  microscopic 
pores,  discharging  the  surplus,  as  excrementitious  matter,  through  their  papillary  orifices. 

131.  Like  the  lungs  of  the  mammalia,  the  leaves  of  trees  are  respiratory  organs,  com- 
posed of  capillary  systems ;  their  mechanical  functions  are  not  so  complete,  though 
their  chemical  functions  may  be  identical.  They  demand  no  nervous  cords  to  be  spread 
upon  them  to  give  them  motion  and  keep  up  their  play ;  the  breezes  in  wbich  they 
tremble  perform  the  office  of  carrying  off  the  exhaled  impurity ;  and  the  rays  of  the  sun 
furnish  them  with  their  vital  force,  enabling  them  to  effect  the  decomposition  of  carbonic 
acid,  and  provide  a  store  of  carbon  for  the  purposes  of  the  economy. 

132.  In  identifying  tbe  mechanical  with  the  chemical  force  of  organic  structures,  we 
see  another  proof  of  that  unity  of  design  existing  through  the  entire  range  of  living 
things.  Functions  of  all  kinds  are  accomplished  by  arrangements  of  every  sort  in  dif- 
ferent classes,  yet  no  one  will  deny  that  they  all  follow  one  original  type.  Digestion, 
as  it  takes  place  in  the  stomach  of  man,  appears  a  highly  complex  phenomenon,  de- 
pending, as  some  say,  partly  on  the  tissue  action,  partly  on  nervous,  and  partly  on  oth- 
er powers.  But  are  not  analogous  changes  wrought  without  all  this  complexity  of  ap- 
paratus in  the  hydatid,  which  may  be  taken  as  the  elementary  type  of  the  stomach ;  or 
in  the  t;enia,  which  is  a  colony  of  stomachs  ?  The  polygastric  infusorials,  some  of 
which  have  hundreds  of  these  organs,  and  even  the  mammalia,  do  not  digest  more  per- 
fectly than  the  hydra,  a  carnivorous  polypus,  which  may  be  turned  inside  out  without 
detriment.  The  laws  of  digestion  followed  by  the  one,  are  followed,  too,  by  the  other. 
If  the  organ  of  the  one  respects  the  presence  of  living  matter,  and  refuses  to  act  on  it,  so 
does  that  of  the  other ;  yet  the  one  is  furnished  with  a  highly  complicated  assemblage  of 
muscular  bands,  of  glandular  apparatus,  of  bloodvessels,  of  nerves,  and  the  other  is  not. 

133.  In  the  higher  orders  of  life,  processes  are  carried  on  by  multiplied  apparatus, 
without,  however,  deviating  from  the  principle  of  the  original  simple  type.    The  gift  of 


/ 


PHYSIOLOGICAL  ILLUSTRATIONS.  42 

a  new  faculty,  or  the  addition  of  a  new  organ,  brings  with  it  a  corresponding  change 
in  the  arrangement  of  the  whole  plan.  An  engineer,  who  wishes  to  adapt  a  machine 
to  the  execution  of  some  new  task,  alters  every  part,  no  matter  how  remote  it  may  be 
from  the  acting  point,  until  every  wheel  and  lever  executes  its  work  co-ordinately  with 
all  the  others  ;  the  prime  mover  remains  unchanged,  though  the  general  character  of  the 
machine  may  have  undergone  a  renovation;  and  as  all  machines,  no  matter  of  how 
many  parts  they  are  composed,  nor  of  how  many  wheels  they  consist,  nor  how  intri- 
cate soever  may  be  their  resulting  motions,  may  have  their  power  reduced  to  and  rep- 
resented by  a  simple  lever,  so  also  organic  functions,  though  often  brought  about  by 
highly  complex  arrangements,  find  simple  representatives  in  the  lower  orders  of  life.  A 
concentration,  or  a  development  of  any  organ,  is  often  demanded  by  change  in  a  re- 
mote part  of  the  fabric,  when  even  the  connexion  may  not  be  very  evident.  Animals, 
consisting  simply  of  digesting  cavities,  require  no  vascular  system  for  propelling  or  con- 
taining a  nutritious  fluid ;  they  are  not  in  need  of  separate  tissues  devoted  to  its  oxyge- 
nation, nor  of  an  insulated  respiration,  nor  do  they  demand  distinct  biliary  organs  ;  when 
the  nutritious  chyle  is  produced  in  the  stomach  of  zoophytes,  it  finds  its  way  into  the 
inter-cellular  spaces,  and  there  circulates  without  vessels,  undergoing  through  the  external 
tegument  the  chemical  changes.  In  many  insect  tribes,  the  bronchial  tubes  are  spent 
upon  the  peritoneum,  and  respiration  takes  place  directly  upon  the  alimentary  canal. 
With  modification  of  functions,  change  of  external  figure  is  always  involved ;  and  as 
these  progress  together,  systems  of  living  things  are  constructed,  referrible  to  one  com- 
mon original  type.  It  is  thus,  in  the  echinodermata,  we  trace  up  successive  steps,  from 
the  sea  urchin  to  the  asterias,  and  from  that  to  the  pentacrinite ;  a  development  of  the 
same  parts  of  the  structure  continually  taking  effect,  until  the  extremes  bear  no  sort  of 
resemblance  to  each  other. 

134.  Had  the  production  of  living  things  been  effected  by  the  operation  of  second 
causes,  we  rnight  look,  with  Lamarck,  for  some  law  of  successive  development  which 
should  contain  the  origin  of  each  order  and  species.  We  might  regard  the  rudimentary 
teeth  of  whales,  or  the  sub-cutaneous  feet  of  the  ophidia,  as  abortive  results  of  such  a  law. 
Considering  the  brain  as  a  development  of  the  spinal  axis,  we  might  trace  in  the  form 
of  the  cranial  bones  a  development  of  a  system  of  vertebra^  brought  about  as  a  conse- 
quence of  the  very  same  laws.  We  might  run  a  parallel  of  analogies  between  the  crus- 
taceous  and  vertebrated  animals,  and  exogenous  and  endogenous  plants  ;  we  might  take 
the  cephalopodous  mollusks  as  furnishing  the  first  rudiments  of  an  internal  skeleton,  and 
trace  its  increasing  complexity  to  meet  certain  ends  until  its  perfect  development  in  the 
mammalia.  In  this  latter  class,  we  might  dwell  upon  the  uniform  existence  of  seven 
cervical  vertebras,  as  giving  evidence  of  a  persistence  in  the  plan  of  structure  in  spe- 
cies so  remote  from  each  other  as  the  camelopard,  the  whale,  and  the  mole.  Parting 
from  the  dorsal  vessel  of  insects,  the  first  rudiments  of  an  aorta,  we  might  follow  out 
the  complications  of  the  higher  arterial  systems.  In  all  the  varieties  of  respiration, 
whether  aquatic,  aerial,  or  mixed,  we  might  see  the  reproduction  of  one  original  chem- 
ical design,  and  in  every  instance  of  a  concentration  of  machinery  or  functions,  we 
might  find  an  impress  of  the  action  of  external  formative  agents. 

F 


4r2 


PHYSIOLOGICAL  RELATIONS  OF  CELLULAR  TISSUE. 


CHAPTER  VI. 

ON   THE    GREAT   MECHANICAL   FORCES    GENERATED   BY   THE    CAPILLARY  ATTRACTION  OP 

CELLULAR  TISSUE. 

{From  the  American  Journal  of  the  Medical  Sciences  for  May,  1838.) 

Contents  :  Pliysiological  Relations  of  Cellular  Tissue. — Foi'ce  ivith  which  Gases  and 
Liquids  jjass  through  Cellular  Tissue. — Disturbing  Action  of  Leakage. —  The  Capil- 
lary Force  overcomes  imwerfiil  Mercurial  Pressure. — Daltons  Hypothesis. —  The  Tis- 
sue is  the  Origin  of  the  Fo?'ce. — Its  Absorbent  and  Condensing  Action. —  Voltaic 
Batteries  may  be  used  for  jjroducing  great  Pressures. — Gases  pass  when  resisted  by 
the  Force  of  many  Atmospheres. —  The  condensed  Gas  acts  as  a  Vacuum. — Co-ordina- 
tion of  the  Results  of  Dalton,  Graham,  and  Mitchell. — Disturbing  Agencies. — Dis- 
turbance by  Variation  of  Temperature. — Physiological  Experiments  and  Remarks. 

135.  Of  all  bodies,  those  alone  are  capable  of  exhibiting  the  phenomena  of  life  which 
consist  of  a  cellular  structure.  Identity  of  chemical  constitution  does  not  appear  to  be 
essential,  yet  it  is  only  a  limited  number  out  of  the  long  list  of  chemical  elements  that  are 
capable  of  organization ;  these,  if  left  alone  to  satisfy  the  conditions  of  their  affinities 
undisturbed,  would  most  commonly  give  rise  to  the  production  of  water,  ammonia,  and 
carbonic  acid.  Life,  therefore,  in  this  point  of  view,  has  no  other  action  than  to  disturb  the 
play  of  these  affinities,  and  force  the  elementary  atoms  into  other  forms  of  combination; 
it  depends  upon  the  success  of  this  action  whether  a  living  or  inorganic  mass  shall 
result.  A  living  body  is  endued  with  a  peculiarity  of  form,  and  does  not  require  an 
identity  of  composition  ;  an  inorganic  body  depends  for  its  nature  on  certain  and  definite 
composition,  without  any  relation  to  structure.  It  is  true  that  most  bodies,  whether  ele- 
mentary or  composite,  exhibit  a  marked  tendency  to  geometrical  arrangements,  and 
all  crystallizations  are  brought  about  by  the  operation  of  polar  forces,  but  an  inorganic 
compound  body  does  not  of  necessity  require  any  peculiar  crystalline  shape,  or  other 
form,  for  existence. 

136.  Life,  then,  is  a  state  of  force;  the  system  of  nature  presents  us  with  but  four 
of  the  chemical  elements  subject  to  it,  for  we  are  taught  to  make  a  distinction  between 
crystalline  arrangement  and  living  structure.  We  have  not  any  direct  evidence  to  show 
that  all  simple  substances  are  in  any  wise  obedient  to  the  laws  of  vitality,  or  that,  when 
they  assume  symmetrical  arrangements  round  an  axis,  that  it  is  an  approach  to  organiza- 
tion, an  imperfect  organization  depending  on  the  sluggishness  of  their  character  or  the 
incompetency  of  the  vital  forces  to  control  the  range  of  their  affinities  ;  nor  is  there  any 
j)p)()f  that  the  laws  directing  the  atomic  arrangement  of  macled  crystals  bear  any  sort 
of  analogy  with  those  that  direct  the  structural  deposite  of  the  radiated  class  of  animals. 
It  is  true  that  the  passage  of  a  polarized  ray  of  light  through  transparent  crystals  has 
disclosed  to  us  the  fact  that  their  atomic  constituents  are  held  together  in  a  state  of 


FORCE  WITH  WHICH  GASES  AND  LIQUIDS  PASS  THROUGH  CELLULAR  TISSUE.  43 

force,  and  we  judge  from  the  phenomena  of  their  nodal  Unes,  when  they  are  thrown 
into  vibration,  that  their  elasticity  varies  in  different  parts ;  yet  the  mere  fact  of  their 
permanence  assures  us  that  they  are  in  a  state  of  stable  equilibrium.  On  the  other 
hand,  organized  structures  are  in  a  condition  of  instable  equilibrium,  and  require  a  con- 
tinued series  of  adjustments  for  the  perpetuation  of  their  existence.  In  the  crystal,  the 
electrical  or  polar  forces  have  compensated  one  another,  and  its  particles  being  brought 
into  a  state  of  rest,  continue  so  without  change;  while  in  the  living  being  their  situation 
is  only  momentary;  they  are  subject  to  incessant  vicissitude  and  change;  their  place 
has  to  be  supplied  by  new  material ;  and  to  accomplish  this  end,  electrical  currents 
traverse  the  body  in  all  directions,  and  machinery  more  or  less  complex  is  employed 
to  bring  new  matter  and  carry  the  effete  away. 

137.  Does  this  cellular  or  areolar  structure,  which  appears  to  be  the  essential  habitat 
of  vitality,  owe  its  properties  to  the  residence  of  a  peculiar  force,  or  are  they  derived 
from  its  organization  1  If  the  latter,  we  ought  to  find  it  possessed  of  remarkable  char- 
acteristics, of  forces  arising  from  the  aggregation  of  particles. 

138.  It  has  been  known  for  some  years  that  gases  and  liquids  pass  through  porous 
structures  with  a  considerable  force.  If  over  the  mouth  of  a  cylindrical  jar  a  thin  sheet 
of  India-rubber  is  tied,  and  the  jar  exposed  to  an  atmosphere  of  annnonia  or  pro- 
toxide of  nitrogen,  in  the  course  of  a  short  time,  by  the  ingress  of  a  portion  of  the  ex- 
ternal atmosphere,  a  pressure  is  created  tending  to  rupture  the  membrane  outward. 

139.  That  the  force  exerted  in  this  case  is  very  great,  appears  from  the  following 
experiment :  In  a  cylindrical  jar,  abed  (_/ig.  17),  four  inches  long,  and  one  and  a  quarter 
in  diameter,  a  siphon  gauge,  e,  was  placed,  and  over  the  mouth  of  the  jar  a  piece  of  In- 
dia-rubber fortified  by  a  layer  of  stout  cloth  was  tied.  Two  pieces  of  tape,  crossing  each 
other  at  the  top  and  passing  down  the  sides  of  the  jar,  were  knotted  as  tightly  as  possible 
at  its  bottom,  and  the  arrangement  was  then  exposed  to  an  atmosphere  of  ammonia. 
In  the  course  of  six  hours,  the  India-rubber,  notwithstanding  it  was  forcibly  held  down 
by  the  cloth  and  tapes,  began  to  stretch  upward,  and  the  gauge  had  risen  thirteen  divisions 
of  an  arbitrary  scale  attached  to  it.  In  twenty-four  hours  it  had  risen  to  nineteen  and 
a  half,  and,  finally,  to  twenty,  after  which  it  remained  stationary.  On  estimating  the 
divisions  of  the  scale,  after  the  experiment  was  over,  it  was  found  that  the  maximum 
pressure  in  this  case  was  about  two  thirds  of  an  atmosphere,  or  ten  pounds  on  the 
square  inch. 

140.  This  effect  is  not  confined  to  gases,  but  takes  place  with  equal  energy  when 
liquids  only  are  used.  In  a  jar  containing  alcohol,  a  gauge  was  placed,  and  a  piece  of 
human  peritoneum  was  stretched  over  the  mouth,  fortified  by  silk.  The  whole  was 
then  sunk  into  a  vessel  of  water.  In  twelve  hours  it  was  found  that  the  level  of  the 
fluid  in  the  gauge  had  risen  the  whole  length  of  the  scale,  and  that  when  the  maximum 
pressure  took  effect,  the  gauge  was  exhibiting  a  condensation  of  one  atmosphere  exactly. 

141.  Here,  then,  we  have  proof  that  the  passage  through  tissues  is  accomplished 
with  a  degree  of  energy  indicating  that  the  forces  which  produce  it  are  of  a  very  high 
intensity.  To  measure  these  forces,  or  to  obtain  some  approximation  of  their  value, 
the  following  researches  were  made. 


44 


DISTURBING  ACTION  OF  LEAKAGE. 


142.  Before,  however,  proceeding  to  the  detail  of  these  experiments,  it  is  necessary 
to  allude  to  certain  disturbing  circumstances  which  take  place,  arising  from  extraneous 
mechanical  action,  and  vitiating  the  result.  One  of  the  most  prominent  of  these  is  due 
to  the  general  leakage  which  happens  through  the  open  pores  of  all  tissues ;  a  leakage 
which  is  to  be  distinguished  from  the  proper  capillary  transit.  If,  for  example,  a  bar- 
rier of  peritoneum  be  placed  over  the  mouth  of  a  vessel  of  water,  under  ordinary  cir- 
cumstances the  escape  of  the  water  will  be  prevented;  but  if  a  pressure  gradually  in- 
creasing be  exerted  on  the  water,  it  will  rapidly  ooze  through  every  pore,  and  finally, 
if  the  membrane  stand  the  strain  without  rupture,  will  spirt  through  those  of  a  large  di- 
ameter. This  effect,  to  a  greater  or  lesser  extent,  takes  place  wherever  tissues  have  to 
resist  mechanical  pressure ;  the  amount  of  disturbance  arising  from  it  depends  mainly 
on  the  diameter  of  the  pores  of  the  structure. 

143.  In  the  experiment  related  in  section  139,  we  have  a  well-marked  instance  of 
this  disturbance.  It  might  be  inferred  from  that  experiment,  that  the  force  with  which 
water  passed  through  a  piece  of  peritoneum  into  alcohol  was  not  greater  than  one  at- 
mosphere ;  whereas,  in  truth,  it  was  much  more ;  but,  as  soon  as  the  pressure  within 
the  vessel  by  the  infiltration  of  water  had  amounted  to  about  one  atmosphere,  the  alco- 
hol escaped  from  the  vessel  as  rapidly  as  the  water  entered,  by  general  leakage  from 
the  whole  surface  of  the  membrane,  and  the  gauge,  therefore,  gave  no  evidence  of  the 
passage  of  the  liquid.  Nor  are  very  porous  structiu'es  alone  liable  to  this  accident;  the 
experiment  of  the  old  Florentine  academicians  shows,  that  even  through  the  pores  of 
gold,  one  of  the  densest  of  the  metals,  fluids  under  a  severe  pressure  will  find  their  way, 
as  appeared  when  they  attempted  to  compress  water  in  a  globe  of  that  metal. 

144.  This  accidental  passage  through  pores  may  be  made  visible  to  the  eye  by  con- 
densing about  one  atmosphere  of  air  into  a  vessel  whose  mouth  is  closed  by  a  sheet  of 
India-rubber,  and  then  placing  it  in  ajar  filled  with  water;  small  bubbles  of  air  will  be 
seen  escaping  from  every  part  of  the  India-rubber,  and  passing  in  great  numbers  through 
the  water. 

145.  It  has  just  been  stated,  that  the  force  with  which  water  passes  through  a  mem- 
brane into  alcohol  is  much  more  than  one  atmosphere;  this  may  be  proved  by  making 
use  of  a  barrier  of  a  stouter  fabric  than  the  peritoneum  here  mentioned.  A  piece  of 
bladder  being  used  in  lieu  of  it,  the  gauge  indicated,  w  hen  the  pressure  was  a  maximum, 
a  force  of  1*8  atmospheres ;  but  even  this  cannot  be  taken  as  the  true  value  of  the 
force,  for  a  certain  period  of  time  elapses,  amounting,  in  this  instance,  to  almost  two 
days,  before  the  gauge  reaches  its  highest  point ;  and  when  that  is  gained,  the  alcohol 
has  become  considerably  diluted,  and  agreeably  to  a  law  which  will  hereafter  be  pointed 
out,  the  amount  of  force  rapidly  diminishes  as  this  takes  effect ;  for,  as  soon  as  the 
composition  of  the  fluids  on  ])oth  sides  of  the  bladder  is  the  same,  provided  the  temper- 
ature of  both  is  alike,  and  no  mechanical  disturbance  arises  from  unequal  pressure,  all 
motion  either  way  ceases,  and  this  may  happen  long  before  the  column  of  fluid  in  the 
gauge  has  reached  its  highest  point. 

146.  The  air  gauge,  however,  at  the  best,  is  a  very  imperfect  indicator  of  the  force 
with  which  gases  or  liquids  mingle,  for  it  will  remain  stationary,  even  when  the  passage 


* 


/ 

THE  CAPILLARY  POWER  OVERCOMES  POWERFUL  MECHANICAL  PRESSURE.  45 

is  taking  place  with  very  great  force,  provided  tlie  rate  of  the  bodies  on  both  sides  of 
the  barrier  is  the  same.  It  gives  erroneous  results  in  all  those  cases  where  the  me- 
chanical leakage  exceeds  the  true  percolation,  and  hence  has  a  very  hmited  application 
in  all  these  experiments.  Other  means  are  therefore  required  to  test  the  passage  of 
fluids,  and  for  this  purpose  there  is  no  arrangement  more  convenient  than  that  represent- 
ed in  fig.  18.  It  consists  of  a  tube,  three  eighths  of  an  inch  in  diameter,  and  several  feet 
long,  bent  at  the  point  b  upward,  and  expanded  at  c  <:  into  a  funnel  termination.  When 
the  instrument  is  in  action,  the  longer  limb,  d  b,  is  filled  to  some  determinate  height  with 
mercury,  which  also  rises  to  a  certain  distance  in  the  shorter  leg,  above  this,  and  to  the 
height  a  d  some  fluid  is  placed  acting  as  chemical  test  of  the  presence  of  the  gas,  in- 
tended to  be  passed  through  the  barrier  c  c,  which  is  tied  air  tight  over  the  funnel 
n)outh.  The  following  experiment  will  indicate  its  use :  Having  placed  this  siphon 
on  the  mercurial  trough,  a  quantity  of  mercury  was  poured  into  it,  sufficient  to  cut  off 
communication  between  the  two  limbs;  then  in  the  shorter  limb  a  column  of  litmus 
water  reddened  by  muriatic  acid,  and  occupying  a  depth  of  one  eighth  of  an  inch,  was 
introduced;  over  the  funnel  mouth  a  thin  lamina  of  India-rubber  was  tied,  and  upon  that 
a  piece  of  stout  silk,  for  the  purpose  of  strengthening  the  barrier.  A  column  of  mercury, 
forty-three  inches  in  height,  was  next  placed  in  the  long  limb,  and  a  jar  of  ammoniacal 
gas  over  the  short  one.  In  the  course  of  one  minute,  a  cloud  of  dark-blue  particles  was 
seen  descending  through  the  litmus,  and  in  six  minutes  it  had  become  uniformly  blue  ; 
thus  proving  the  passage  of  ammonia  through  a  tissue  of  India-rubber,  against  a  pressure 
of  almost  one  atmosphere  and  a  half. 

147.  There  were  considerable  difficulties  encountered  in  the  outset  of  these  experi- 
ments in  tying  on  the  India-rubber  barriers,  so  as  to  withstand  the  high  pressures  to 
which  they  were  exposed  without  leakage ;  an  insidious  leakage  which  took  place  be- 
tween the  sides  of  the  glass  and  that  part  of  the  India-rubber  compressed  by  the  string 
against  it.  This,  however,  was  effectually  prevented  by  setting  fire  to  a  piece  of  India- 
rubber,  and  daubing  the  semi-fluid  material  on  that  part  of  the  glass  around  which  the 
string  was  to  pass  ;  then,  on  tightly  binding  on  the  barrier,  it  came  into  perfect  contact 
with  the  glass,  and  was  retained  there  by  the  sticky  material,  no  leakage  whatever  taking 
place,  unless  some  part  of  the  arrangement  burst. 

148.  The  experiment  just  related  leads  to  some  important  conclusions  ;  we  see  that 
the  force  of  impulsion  driving  ammonia  into  atmospheric  air  exceeds  a  pressure  equiv- 
alent to  forty-three  inches  of  mercury,  the  barometric  pressure  at  the  time  being  29-73, 
that  is  to  say,  exceeding  by  very  near  half  an  atmosphere  the  force  which  theory 
would  indicate.  The  hypothesis  of  Mr.  Dalton,  which  seems  to  me  to  be  fully  con- 
firmed by  the  observations  of  Mr.  Thomson,  founded  on  the  experiments  of  Mr.  Gra- 
ham, assumes  that  gases  act  towards  each  other  as  vacua,  or,  in  other  words,  the  force 
impelling  the  particles  of  one  gas  into  the  intersticeiS  of  another  does  not  exceed  the 
barometric  pressure  ;  but  here  we  find  that  the  result  apparently  leads  to  a  very  differ- 
ent conclusion.  It  was  from  an  experiment  of  this  kind  that  Dr.  J.  K.  Mitchell  was 
led  to  doubt  the  truth  of  Dalton's  theory,  inferring  from  his  results  that  gases  penetrated 
each  other  with  much  greater  force.    Such  a  conclusion,  however,  does  not  legitimately 


46 


DALTON'S  HYPOTHESIS. 


follow,  for  it  is  higlily  probable  that  the  nature  of  the  barrier  itself  is  very  much 
concerned  in  the  final  action.  A  gas  may  penetrate  into  another  with  a  force  not 
greater  than  one  atmosphere,  and  yet,  because  of  the  disturbing  agency  of  the  medium 
through  which  it  must  go,  it  may  succeed  in  lifting  a  column  of  mercm-y  equivalent  to 
a  pressure  of  many  atmospheres. 

149.  The  evidence  proving  that  gases  do  not  infiltrate  each  other  with  a  pressure 
greater  tban  one  atmosphere  is  very  cogent.  Much  of  its  weight  is  derived  from  the 
identity  of  the  resulting  volumes  of  commingled  gases  ;  but  the  most  important  fact  re- 
lates to  the  passage  of  these  substances  into  each  other,  when  the  barrier  separating 
them  is  very  porous  and  has  no  condensing  action,  as  is  the  case  with  a  stucco  plug, 
which  opposes  simply  a  mechanical  impediment  to  their  motion,  acting,  as  will  be  here- 
after proved,  merely  as  a  temporary  valve ;  a  mode  of  action  totally  different  to  that  of 
closer  textures.  The  final  volumes  exchanged  being  inversely  proportional  to  the 
square  root  of  the  densities,  and  these  final  volumes  representing  the  true  initial  veloci- 
ties, we  have  a  striking  illustration  of  that  law  of  gaseous  mechanics,  that  the  velocities 
of  different  gases,  rushing  into  a  vacuum,  are  inversely  proportional  to  the  square  root 
of  their  densities.  Consequently,  we  are  constrained  to  infer  that  one  gas  acts  towards 
another  in  the  same  manner  as  if  it  were  a  vacuum  ;  and,  therefore,  that  the  force  im- 
pelling the  particles  of  one  gas  into  the  interstices  of  another  never  exceeds  the  pres- 
sure of  one  atmosphere. 

150.  In  an  experiment  made  on  the  passage  of  ammonia  into  atmospheric  air,  it  was 
found,  that  though  the  passage  of  the  gas  was  resisted  by  a  pressure  of  seventy-five  inches 
of  mercury,  or  upwar4  of  two  atmospheres  and  a  half,  it  took  place,  apparently,  as 
readily  as  if  no  such  resistance  had  been  opposed  to  it.  The  question  at  once  arises, 
Whence  is  this  powerful  impulsive  force  derived  1  clearly  not  from  the  action  of  one 
gas  upon  the  other,  for  there  is  great  probability,  as  we  have  already  seen,  that  that 
force  would  not  be  able  to  lift  more  than  thirty  inches  of  mercury.  The  porous  tissue 
or  barrier  alone  can  be  regarded  as  the  seat  of  this  power.  This  fact,  that  systems  of 
capillary  tubes,  or  thin  tissues,  have  in  themselves  certain  powers,  capable  of  producing 
high  mechanical  action,  and  operating  successfully  against  the  severest  pressures  that 
can  be  brought  to  bear  against  them,  is  worthy  of  the  serious  contemplation  of  physiol- 
ogists ;  it  is  a  great  error  to  impute  the  forces  producing  these  phenomena  to  the  gas- 
eous media.  In  the  tissue  itself  we  must  admit  a  source  of  power,  a  source  far  trans- 
cending that  which  solicits  the  gases  to  penetrate  each  other.  Let  us  next  inquire 
into  the  nature  of  this  power. 

151.  It  is  well  known  that  porous  substances  of  all  kinds  and  fluids  absorb  gaseous 
matter  very  readily,  in  volumes  varying  according  to  circtimstances.  Water,  for  exam- 
ple, absorbs  its  own  volume  of  carbonic  acid,  and  480  times  its  volume  of  hydrochloric 
acid  gas.  In  the  latter  case,  therefore,  an  extremely  great  condensation  takes  place. 
So,  too,  a  fragment  of  porous  charcoal  absorbs  nearly  ten  times  its  volume  of  oxygen, 
and  ninety  times  its  volume  of  ammonia ;  these  gases,  therefore,  exist  on  the  surface  of 
the  particles  of  the  absorbing  medium,  in  a  state  of  very  high  compression.  And  the  rea- 
soning which  here  applies,  applies  also  in  the  case  where  the  two  gases  are  separated  by 


ABSORBENT  AND  CONDENSING  ACTION  OF  BODIES. 


a  tissue.  If,  for  example,  we  separated,  by  a  medium  of  this  kind,  a  certain  volume  of 
ammonia  from  a  like  volume  of  nitrogen  gas,  though  at  the  outset  of  the  experiment 
both  the  gases  might  be  existing  under  the  same  pressure,  yet  this  equality  would  very 
rapidly  be  lost.  The  absorption  of  the  ammonia  taking  place  with  much  more  rapidity 
than  the  nitrogen,  it  would  be  presented  to  this  latter  gas,  not  under  an  equivalent  pressure, 
but  in  a  state  of  great  condensation.  Under  such  circumstances,  the  transit  of  a  gas 
is  not,  as  will  be  shortly  shown,  analogous  to  the  case  where  it  flows  under  common 
pressure  into  a  vacuum,  or  into  another  gas,  but  the  tissue,  continually  acting  as  a  per- 
petual condensing  engine,  brings  the  two  media  in  contact  with  each  other  under  ex- 
tremely different  conditions ;  the  one  in  a  compressed  state,  but  ready  to  exert  the 
whole  of  its  elastic  force,  the  other  in  a  state  perhaps  little  varying  from  its  normal 
condition. 

152.  If  tissues  really  exert  a  power  of  this  kind,  some  might  inquire  how  it  is  that, 
when  a  tube  closed  at  one  end  with  such  a  structure,  and  filled  with  mercury,  is  sunk 
in  the  trough  to  its  hydrostatic  level,  atmospheric  air,  or  any  gas  to  which  it  is  ex- 
posed, does  not  pass  through  and  expel  the  mercury  from  the  tube.  If,  it  might  be 
said,  the  gas  is  existing  in  such  a  condensed  state  in  the  tissue,  what  is  the  reason  it 
does  not  expand,  and  drive  the  mercury  down  ?  Experiment  proves  that  this  is  not  the 
case,  but  no  argument  can  be  drawn  from  it  at  all  affecting  the  position  here  taken ; 
for,  as  soon  as  the  gas  has  gained  the  under  side  of  the  tissue,  there  is  no  cause 
soliciting  it  to  escape  any  more  this  way  than  backward  into  its  own  atmosphere ; 
the  pressures  each  way  are  equal,  and,  therefore,  counteract  each  other's  effects ;  or, 
rather,  the  pressures  are  unequal,  for  that  tending  to  expel  the  mercury  is  resisted  by  the 
hydrostatic  action  of  that  fluid,  and  hence  no  gas  can  pass  into  the  tube. 

153.  We  can  now  understand  the  rationale  of  action  in  Mr.  Graham's  experiment 
with  plugs  of  stucco.  He  found  that  this  material  exerted  a  very  slight  absorbent 
power  over  the  gases ;  oxygen,  hydrogen,  nitrogen,  &c.,  not  being  absorbed  in  any 
sensible  quantity.  When,  therefore,  he  diffused  hydrogen  into  atmospheric  air,  the 
stucco  not  acting  mechanically  on  either  of  those  substances,  they  were  presented  to 
each  other  under  equal  and  ordinary  pressures,  and  they  therefore  began  to  flow  into 
each  other  just  in  the  same  way  that  they  would  have  flowed  into  a  vacuum ;  but  very 
different  is  the  result  when  we  make  use  of  sheets  of  India-rubber  or  moistened  animal 
membranes.  The  stucco  plug  serves  only  to  make  the  experiment  manageable  by  op- 
posing a  slight  resistance  to  the  escape  of  the  gases,  and  acting,  as  I  have  said  before 
as  a  temporary  valve ;  so  that,  if  a  diffusion  tube  be  fitted  up  in  Mr.  Graham's  manner, 
at  the  end  of  the  arm  of  a  balance,  the  gas  does  not  escape  so  rapidly  but  that  there  is 
time  for  a  very  accurate  self-adjustment  of  the  apparatus,  and  the  volume  of  re-entered 
air  can  be  measured  with  precision. 

154.  It  might,  perhaps,  be  objected  to  the  view  here  taken,  that  the  condensation 
which  some  gases  experience  is  more  than  sufficient  to  liquefy  them;  and  that,  there- 
fore, they  do  not  act  simply  as  gaseous  bodies  would  do  towards  each  other.  This 
condition,  however,  when  it  does  take  place,  appears  not  to  change  the  resulting  plie- 
nomena,  as  the  following  experiment  shows.    The  thermometer  being  at  38°  F.,  and 


48         VOLTAIC  BATTERIES  MAY  BE  USED  FOR  PRODUCING  GREAT  PRESSURES. 


the  barometer  at  29-88,  atmospheric  air,  under  a  pressure  of  two  atmospheres  and  a 
half,  was  exposed,  under  a  sheet  of  India-rubber,  to  sulphurous  acid  gas,  care  being  taken 
that  the  temperature  of  the  mercurial  trough,  and  all  parts  of  the  arrangement,  should 
be  as  above.  The  passage  of  the  gas  took  place  with  great  promptness,  the  litmus 
water,  used  to  detect  its  presence,  reddening  rapidly.  Now  sulphurous  acid,  according 
to  the  experiments  of  Dr.  Faraday,  condenses  into  a  liquid  at  45°  F.,  under  a  pressure 
of  thirty  inches  of  mercury  ;  we  know,  therefore,  that  in  this  trial  the  gas  must  have 
existed  in  a  liquid  condition  in  the  barrier,  and  yet  it  passed  through  into  atmospheric 
air,  under  a  resistance  almost  two  and  a  quarter  times  sufficient  to  condense  it,  and  at 
a  temperature  eight  degrees  lower. 

155.  Having  progressed  thus  far  in  this  part  of  the  inquiry  on  the  action  of  tissues, 
it  became  important  to  find  if  any  pressure  which  could  conveniently  be  brought  into 
action  would  restrain  the  passage  of  gaseous  matter.  Resort  was  first  had  to  the  usual 
mechanical  condensing  apparatus ;  but  they  were  found  to  be  ill  adapted  to  the  purpose 
in  hand.  The  necessary  motions  were  always  productive  of  inconvenience,  and  it  was 
not  found  possible  to  carry  the  condensation  to  the  degree  required,  or  to  avoid  leak- 
age from  some  of  the  numerous  joints.  After  some  trouble,  the  following  contrivance 
was  fallen  upon,  which  answers  the  end  perfectly,  is  not  open  to  the  serious  objections 
of  the  former,  and,  requiring  no  cock  or  valve,  can  be  readily  made  without  leakage.  A 
tube  of  glass  about  one  third  of  an  inch  in  bore,  of  stout  substance,  and  about  ten  inches 
long,  is  bent  into  a  kind  of  siphon,  so  that  one  leg  shall  be  about  six,  and  the  other 
two  inches  long.  The  extremity,  a  a,  jig.  19,  has  a  lip  or  rim  turned  round  it  at  the 
lamp ;  while  in  the  longer  leg,  a  thin  glass  tube,  c  c,  about  one  eighth  of  an  inch  in 
bore,  and  closed  at  one  end,  is  included  to  serve,  as  will  be  hereafter  shown,  as  a  gauge. 
Next,  the  extremity,  h,  of  the  siphon  is  closed,  there  being  inserted  through  it  two  pla- 
tinum wires,  d  d,  e  e,  parallel  to  each  other,  but  not  touching.  The  arrangement  is  then 
ready  for  use.  Suppose,  for  example,  it  was  required  to  pass  through  India-rubber 
sulphurous  acid  gas  into  atmospheric  air,  condensed  by  a  pressure  of  five  or  six  atmo- 
spheres ;  the  long  leg  of  the  siphon  is  to  be  filled  with  water,  which  is  excluded  from 
the  gauge-tube  c  c,  owing  to  the  narrowness  of  its  bore  ;  next,  a  strong  decoction  of  lit- 
mus is  to  be  poured  into  the  short  leg  until  it  is  about  half  filled.  The  rim  round  the 
extremity,  a  a,  is  then  daubed  with  a  piece  of  burning  caoutchouc,  and  upon  it  is  tied 
a  thin  piece  of  that  substance,  with  a  fine  but  strong  waxed  thread.  Over  this  is  tied 
a  piece  of  stout  silk  or  cotton  cloth,  for  the  purpose  of  fortifying  the  barrier;  the  wires, 
d  d,  e  e,  are  then  made  to  communicate  with  the  poles  of  an  active  voltaic  battery,  and 
the  condensation  commences ;  for  the  gas  which  is  evolved  from  these  electrodes  rising 
to  the  top  of  the  tube,  accumulates  there,  causing  the  column  of  water  in  the  short  leg  to 
rise  and  condense  the  atmospheric  air  above  it.  The  membrane,  though  fortified,  gives 
way  to  a  certain  extent,  becoming  convex  outward;  and  as  the  accumulation  of  gas  in 
the  long  leg  continues,  the  condensation  of  that  in  the  short  leg  increases,  as  is  indica- 
ted by  the  gauge  c  c.  A  very  thin  India-rubber,  of  the  diameter  here  used,  will  stand  a 
pressure  of  from  six  to  twenty  atmospheres  without  rupture,  if  its  silk  support  is  good ; 
and  I  have  found  that  anointing  the  edges  of  the  rim  with  the  burned  substance  enables 


I 

VOLTAIC  BATTERIES  MAY  BF  USED  FOR  PRODUCING  GREAT  PRESSURE.  49 

the  operator  to  tie  it  on  so  that  110  leakage  shall  occur  between  the  India-rubber  and 
the  glass,  even  under  the  severest  pressures.  When  the  gauge  shows  that  the  required 
degree  of  condensation  is  arrived  at,  the  connexion  with  the  battery  is  broken,  and  the 
condensation,  of  course,  stops;  the  siphon  being  carried  to  the  mercurial  trough,  taking 
care  to  keep  its  position  erect,  its  short  limb  is  depressed  under  the  mercury,  and  car- 
ried into  a  jar  containing  the  sulphurous  acid.  If,  under  these  pressures,  any  of  the 
acid  gas  finds  its  way  into  the  condensed  air,  its  presence  is  detected  by  the  reddening 
of  the  blue  litmus  water.  It  is  necessary  here  to  observe,  that  the  indications  of  the 
air  gauge  do  not  give  a  correct  estimate  of  the  amount  of  condensation,  but  always  rep- 
resent them  higher  than  they  are  according  to  Marriotte's  law :  it  has  long  been  known 
that  the  volume  of  gas  dissolved  by  water  depends,  in  a  great  measure,  on  the  pressure 
exerted  on  it :  now  it  will  be  found,  when  the  operation  is  conducted  in  an  instrument 
arranged  as  this,  that  a  very  large  proportion  of  the  air  in  the  gauge  disappears  in  this 
manner;  its  zero  point  is  therefore  altered,  and  the  condensation  appears  higher  than  it 
really  is.  It  may  be  remarked,  in  passing,  that  it  is  surprising  to  what  an  extent  the 
absorption  of  oxygen  and  hydrogen  is  carried  in  the  longer  leg,  owing  to  their  making 
their  appearance  in  a  nascent  form.  To  ascertain  the  true  condensation,  so  soon  as 
the  passage  of  the  sulphurous  acid  or  other  gas  has  taken  place  satisfactorily,  the  mem- 
brane is  to  be  punctured  with  a  pin,  and  when  a  pneumatic  equilibrium  is  obtained,  the 
height  of  the  liquid  in  the  gauge  will  mark  the  point  where  the  zero  of  the  scale  should 
be  placed. 

156.  Some  might  suppose  that  there  is  danger  in  making  use  of  an  apparatus  like 
this,  where  a  high  pressure  is  produced,  owing  to  the  risk  of  an  explosion  of  the  com- 
pound gases  in  the  long  limb,  since  it  is  stated  in  most  works  on  chemistry  that  a  mix- 
ture of  oxygen  and  hydrogen,  when  compressed,  will  explode.  To  ascertain  if  there 
was  any  danger  arising  from  this,  as  also  to  know  to  what  extent  the  condensation  could 
be  pushed  by  the  aid  of  a  voltaic  battery,  I  took  a  tube,  a  h  {Jig.  20),  and  into  the  closed 
extremity  having  fused  a  pair  of  platinum  wires,  and  drawn  the  other  into  a  long  capil- 
lary tube,  bending  it  at  the  same  time  at  right  angles  to  the  former,  I  filled  it  with  water 
(boiled  until  all  the  air  mechanically  enclosed  in  it  was  expelled),  except  a  portion  of  the 
narrow  capillary  part  from  cl  to  c,  which  contained  atmospheric  air,  to  act  as  a  gauge  ; 
the  extremity,  c,  was  closed.  Next  the  platina  wires  were  made  to  communicate  with 
the  poles  of  an  active  voltaic  battery  of  120  pairs,  and  gas  slowly  accumulated,  the  cur- 
rent of  electricity  steadily  passing  all  the  time,  as  was  indicated  by  the  deviations  of  a 
galvanometer,  through  which  it  was  made  to  circulate.  Observations  were  made  every 
few  minutes  on  the  progress  of  the  experiment,  the  last  of  which  indicated  a  pressure  of 
slightly  upward  of  forty-three  atmospheres,  and  shortly  after  it  was  taken  the  tube  burst; 
not,  however,  on  account  of  the  explosion  of  the  gaseous  materials  in  it,  but  because  it 
could  not  sustain  so  excessive  a  pressure  tending  to  burst  it,  a  pressure  equivalent  to  that 
of  a  column  of  mercury  nearly  thirteen  hundred  inches  high. 

157.  These  results  lead  us  to  some  remarkable  conclusions  in  relation  to  the  passage 
of  voltaic  currents.  Dr.  Faraday  found  that  they  cannot  pass  along  such  media  as 
water  without  effecting  its  decomposition  ;  in  fact,  that  the  transfer  of  elements  seemed 

G 


50 


GASES  PASS  WHEN  RESISTED  BY  THE  FORCE  OF  MANY  ATMOSPHERES. 


to  be  absolutely  essential  to  the  transit  of  the  electricity.  Now  it  might  be  supposed 
that,  if  some  powerful  force  were  brought  to  bear  against  and  antagonize  this,  as  where, 
by  a  severe  pressure,  the  oxygen  and  hydrogen  are  prevented  from  being  evolved,  one  of 
four  things  must  happen :  1st.  That  the  water  would  become  a  non-conductor.  2(1. 
That  the  vessel,  no  matter  how  strong  it  might  be,  would  burst.  3d.  That  the  current 
would  pass  without  any  decomposition  happening;  or,  lastly,  that  the  current  would 
pass  and  gas  be  evolved,  but  as  fast  as  evolved,  it  would  be  dissolved  in  the  water.  A 
quantity  of  boiled  water  was  hermetically  sealed  up  in  a  glass  tube,  which  it  filled  en- 
tirely, except  a  small  space  occupied  by  a  bubble  of  air,  probably  not  more  than  one 
fiftieth  part  of  an  inch  in  diameter.  A  pair  of  platinum  wires  had  been  fixed  into  the 
tube  so  as  to  transmit  the  voltaic  current.  The  current  passing  freely,  as  was  indicated 
by  a  galvanometer,  decomposition  of  the  water  ensued;  extremely  minute  bubbles  making 
their  appearance,  the  water  absorbing  the  greatest  part  of  them,  its  temperature  rising 
very  much,  so  that  the  tube  counnunicated  a  sensation  of  warmth  when  touched  by  the 
finger.  When  the  pressure  was  estimated  to  have  risen  to  about  fifty  atmospheres,  the 
tube  burst,  and  in  an  instant  all  the  gas  that  had  been  imprisoned  in  the  water  made  its 
escape,  throwing  it  into  a  violent  effervescence.  Hence  we  find,  that  when  water  is 
enclosed  hermetically  in  a  vessel,  and  a  galvanic  current  passes  through  it,  decomposition 
ensues,  a  portion  of  the  gases  making  their  appearance  in  a  gaseous  form,  replacing  the 
small  space  occupied  by  the  decomposed  water,  the  whole  of  the  remainder  being  ab- 
sorbed by  that  fluid  as  fast  as  it  is  given  off.  When  the  pressure  is  high,  it  is  probable 
that  the  dimensions  of  the  vessel  become  greater,  and  hence  the  little  bubble  of  air  ac- 
cumulated exceeds  in  bulk  the  volume  of  decomposed  water.  It  is  also  found  that  any 
pressure  up  to  forty  or  fifty  atmospheres  may  be  commanded  in  this  way. 

158.  Being  thus  furnished  with  a  very  convenient  and  very  portable  method  of  con- 
densation, I  proceeded  to  examine  the  force  of  passage  of  gaseous  matter  into  atmo- 
spheric air.  Sulphurous  acid  passed  instantaneously  into  atmospheric  air,  against  a 
pressure  equivalent  to  two  hundred  and  twenty  inches  of  mercury,  or  seven  atmospheres 
and  a  third.  Some  experiments  were  made  on  the  absorbing  action  of  the  sample  of 
India-rubber  here  used,  which  had  been  softened  in  ether  for  the  purpose  of  procuring 
it  in  tiiin  sheets.  Of  the  gas  here  spoken  of,  it  was  found  to  absorb  sixteen  times  its 
own  volume.  It  is  to  be  expected  that,  even  had  a  much  more  powerful  pressure  been 
applied,  the  gas  would,  nevertheless,  have  gone  through. 

]  59.  The  curved  form  of  the  instrument  described  in  (155)  was  found  to  present  cer- 
tain inconveniences  when  pressures  upward  of  six  or  seven  atmospheres  were  made  use 
of;  the  volume  of  air  which,  at  the  beginning  of  the  experiment,  occupied  the  greater 
part  of  the  extent  of  the  shorter  limb,  had  now  collapsed  much  in  its  dimensions,  and 
owing  to  the  unavoidable  giving  way  of  the  India-rubber  and  its  silk,  had  retreated  out 
of  sight  beneath  it.  It  was  not  found  convenient  to  lengthen  this  limb,  for  that  entailed 
a  corresponding  increase  in  the  dimensions  of  the  battery,  in  order  to  produce  a  given 
condensation  in  a  given  time  ;  an  objection  also  applying,  in  a  measure,  to  the  apparatus 
even  at  lower  pressures.  Though  I  had  the  conmiand  of  batteries,  consisting  of  six 
hundred  pairs  of  four-inch  plates,  I  preferred  a  modification  in  the  instrument  itself,  than 


GASES  PASS  WHEN  RESISTED  BY  THE  FORCE  OF  MANY  ATMOSPHERES.  51 

a  resort  to  such  an  energetic  but  unwieldy  apparatus.  A  straight  tube  was,  therefore, 
taken,  about  three  sevenths  of  an  inch  in  bore  {Jig.  21),  and  a  rim  turned  on  it  at«a; 
at  the  closed  extremity  the  platina  wires,  h  c,  entered,  a  gauge  tube,  dd,  was  dropped  in 
between  them,  water  was  then  poured  in  to  the  height,  e  e,  and,  lastly,  a  tubc,yj  con- 
taining the  appropriate  chemical  test,  was  inserted,  its  bottom  resting  on  the  top  of  the 
gauge  tube.  Nothing  then  remained  but  to  tie  on  the  India-rubber,  with  its  silken  sup- 
port, and  by  the  voltaic  battery  to  proceed  to  condense.  In  this  instrument,  the  test  fluid 
was  never  out  of  sight,  nor  did  the  volume  of  the  gas  suffer  any  inconvenient  change; 
the  gauge,  too,  was  well  located  for  observation,  and  a  given  condensation  could  be  pro- 
duced in  less  time,  and  by  a  less  amount  of  electricity,  than  with  the  siphon  tube  ;  for 
the  space  contained  between  a  a  and  e  e  was  less  in  volume.  As  an  auxiliary  arrange- 
ment, a  glass  tube, aaaa  {  fig.  22),  one  inch  in  diameter  and  ten  long,  with  a  support,  />, 
was  taken,  and  its  mouth  ground  true,  so  that  a  piece  of  plate  glass,  e  c,  would  close  it  when 
placed  over  it ;  this  tube  served  in  many  cases  as  a  gas  generator,  and  also  as  a  receiver 
for  the  tube  {fig.  22),  which  was  dropped  into  it.  It  is  to  be  observed,  that  in  the  ar- 
rangement here  adopted,  the  gaseous  matter  evolved  from  water  mingles  with  the  atmo- 
spheric air  in  the  upper  part  around  the  tube,  /  and,  therefore,  the  passage  of  the  gases 
ti-ied  does  not  take  place  into  atmospheric  air,  but  into  a  mixture  of  oxygen,  hydrogen, 
and  nitrogen  gases. 

160.  The  tube  /"being  filled  with  lime-water,  and  a  pressure  amounting  to  ten  atmo- 
spheres being  produced  in  the  vessel,  it  was  exposed  to  an  atmosphere  of  carbonic  acid, 
generated  in  the  tube  aa  a  a,  fig.  22,  procured  by  dropping  a  few  pieces  of  marble  into 
the  tube,  and  pouring  thereon  dilute  muriatic  acid.  When  the  vessel  was  full,  the 
plate  e  c  was  laid  upon  it,  and  any  surplus  gas  generated  escaped  by  lifting  it  up.  In 
the  course  of  a  few  minutes,  the  upper  part  of  the  tube  containing  lime-water  began  to 
look  milky,  and  in  an  hour,  a  cloud  of  particles  of  carbonate  of  lime  had  fallen  to  the 
bottom. 

161.  Again,  having  filled  the  tube  /  with  a  solution  of  acetate  of  lead,  and  produced 
a  pressure  amounting  to  twelve  atmospheres,  it  was  exposed  to  sulphuretted  hydrogen, 
generated  in  the  vessel, ^o^.  22,  from  protosulphuret  of  iron  and  dilute  sulphuric  acid. 
In  a  very  short  time,  the  black  sulphuret  of  lead  appeared,  giving  tokens  of  the  rapid 
passage  of  this  gas  through  the  barrier.  A  comparative  experiment  was  made,  in  order 
to  discover  whether  the  transmission  took  place  more  slowly  than  when  it  was  not  re- 
sisted by  such  a  severe  pressure.  It  appeared,  however,  so  far  as  the  experiment  could 
be  tried  under  similar  circumstances,  as  regards  the  thickness  of  the  barrier,  *fec.,  that 
sulphuretted  hydrogen  went  through  the  barrier  against  a  pressure  of  three  hundred  and 
sixty  inches  of  mercury,  as  readily  as  if  no  such  force  were  exerted  against  it. 

162.  As  numerous  experiments,  which  had  been  tried  on  various  gases,  had  hitherto 
failed  to  indicate  any  obstacle  to  their  passage,  it  became  necessary  to  know  whether, 
at  the  extremest  pressures  that  could  be  commanded,  they  would  pass  through  a  bar- 
rier. To  accomplish  this,  I  took  a  strong  and  narrow  tube,  and,  having  turned  a  rim 
at  one  end  and  sealed  fine  platina  wires  in  the  other,  I  filled  it  with  distilled  water,  and 
enclosed  in  it  a  narrow  capillary  tube,  the  gaseous  contents  of  which  were  small.  As 


52 


GASES  PASS  WHEN  RESISTED  BY  THE  FORCE  OF  MANY  ATMOSPHERES. 


a  test,  in  the  upper  part  of  the  arrangement,  and  in  Heu  of  the  tube  /  I  placed  a  shp 
of  paper,  which  had  been  alternately  soaked  in  acetate  of  lead  and  cai'bonate  of  soda ; 
the  India-rubber  was  fortified  by  a  piece  of  very  strong  silk,  which  was  carefully  tied 
on;  there  was  not,  therefore,  any  gaseous  matter  present  except  the  small  quantity  of 
atmospheric  air  in  the  gauge-tube.  The  condensation,  therefore,  went  on  with  great 
rapidity,  a  mixture  of  oxygen  and  hydrogen  gradually  accunmlating  in  the  top  of  the 
vessel,  bulging  out  the  India-rubber  and  silk  barrier  until  it  was  almost  hemispherical. 
It  was  my  intention  to  try  a  pressure  of  twenty-five  atmospheres;  and  when  that  was 
supposed  to  be  reached,  the  instrument  was  placed  in  an  atmosphere  of  sulphuretted 
hydrogen.  Very  soon  the  test  paper  became  of  a  tawny  appearance,  and,  finally,  it  was 
quite  black.  The  pressure,  when  the  experiment  was  over,  was  found  to  be  twenty- 
four  and  a  quarter  atmospheres. 

163.  At  a  temperature  of  48°  F.,  and  pressure  of  29-74  B.,  sulphuretted  hydrogen 
gas  passes  into  a  mixture  of  oxygen  and  hydrogen,  though  it  may  be  resisted  by  a  pres- 
sure of  twenty-four  and  a  quarter  atmospheres,  or  nearly  seven  hundred  and  thirty 
inches  of  mercury.  Like  sulphurous  acid,  it  penetrates  through  a  barrier,  and  then  dif- 
fuses into  an  atmosphere  beyond  it,  at  pressures  greater  than  that  which  is  necessary 
to  condense  it  into  a  liquid. 

164.  If,  as  it  thus  appears,  no  pressure  which  we  can  command  is  sufficient  to  re- 
strain one  gas  from  passing  into  another,  we  next  inquire  what  obstacle  the  condensed 
gas  exhibits.  There  is  abundant  and  conclusive  evidence  that,  under  ordinary  circum- 
stances of  temperature  and  pressure,  this  medium  bears  the  same  relation  to  the  perco- 
lating gas  that  a  vacuum  would  do  ;  inasmuch  as  the  rate  of  discharge  into  it  is  iden- 
tically the  same  as  it  is  into  a  vacuum.  For  the  purpose  of  illustration,  we  may,  there- 
fore, regard  it  to  all  intents  as  a  vacuum,  and  reason  accordingly.  If  the  particles  of 
heterogeneous  gases  possess  no  repulsive  tendency  as  respects  each  other,  but  are  per- 
fectly quiescent  and  neutral ;  if  the  presence  or  absence  of  one  makes  no  difference  nor 
produces  any  retardation  on  the  motions  of  the  particles  of  the  other,  then  it  is  appa- 
rent that  it  is  immaterial  how  many  of  such  particles  are  condensed  together  into  a  given 
space  ;  owing  to  the  want  of  repulsive  action  in  those  particles,  that  space  will  be  as 
much  a  vacuum  to  any  other  gas  as  it  ever  was.  Now  it  has  been  shown  by  the  ex- 
periment above  cited,  that  certain  gases  will  diffuse  into  others,  even  though  the  latter 
may  be  condensed  into  a  space  twenty-four  times  less  than  that  which  they  would  or- 
dinarily occupy.  The  vacuum  is  not  less  a  vacuum  because  it  is  contained  under  smaller 
dimensions,  any  more  than  a  torricillian  vacuum  is  less  perfect  when  the  mercury  is 
made  to  rise  nearly  to  the  top  of  the  harometric  tube,  than  it  was  ivhen  there  was  a  va- 
cant space  many  inches  in  length.  Theory  would  therefore  indicate  that  these  diffusions 
will  take  place  under  all  pressures,  provided  the  gaseous  condition  subsists;  and  this 
conclusion  is  abundantly  borne  out  by  the  experiments  herein  detailed. 

165.  Having  thus  shown  how  it  is  that,  when  gaseous  matter  is  on  one  side  of  a 
barrier,  the  space  so  occupied  may  be  regarded  as  a  vacuum,  even  though  the  gas  should 
be  highly  condensed,  I  come  next  to  the  consideration  of  a  much  more  intricate  part  of 
the  subject,  the  action  of  the  barrier  itself  as  an  areolar  tissue,  which  is  the  more  imme- 


CO-ORDINATION  OF  DALTON'S,  GRAHAM'S,  AND  MITCHELL'S  RESULTS.  53 

diate  object  of  this  paper.  I  have  ah-eadj  stated  that  the  resuks  of  Dr.  Mitchell  and 
Professor  Graham  apparently  exhibit  a  striking  discordance  ;  it  will  here  be  seen  that 
the  facts  reported  by  those  chemists  can  be  readily  co-ordinated. 

16G.  Both  of  them  appear  to  have  made  trials  of  the  absorbent  power  of  the  barriers 
they  respectively  employed  ;  Professor  Graham  having  operated  on  a  mass  of  stucco 
of  certain  dimensions,  and  found  its  absorbing  power,  in  relation  to  most  gases,  very 
low  ;  Dr.  Mitchell  on  a  thick  cylinder  of  gum  elastic ;  but  neither  of  them  appears  to 
have  clearly  seen  the  importance  of  this  element  in  the  production  of  the  final  result. 
In  the  case  of  the  action  of  stucco,  this,  indeed,  is  a  remarkable  circumstance,  for  in  all 
those  instances  where  the  absorbing  power  of  the  stucco  was  great,  the  equivalent  vol- 
umes of  diffusion,  as  obtained,  were,  without  exception,  erroneous.  Dr.  Mitchell,  on 
seeing  certain  gases  pass  into  each  other  with  a  force  that  was  greater  than  the  pres- 
sure of  sixty-three  inches  of  mercury,  and  inferring  that  there  was  no  vis  a  tergo  in 
play,  was  obliged  to  impute  his  result  to  the  inherent  power  of  gaseous  penetration; 
hence  he  came  into  direct  collision  with  the  Daltonian  hypothesis.  On  the  other  hand. 
Professor  Graham,  supposing  that,  in  all  his  erroneous  cases,  the  deficit  was  to  be  im- 
puted to  the  porous  mass,  which,  in  some  manner,  detained  and  absorbed  the  gases, 
found  in  every  other  instance  a  full  confirmation  of  the  doctrine  of  a  vacuum. 

1G7.  The  whole  phenomenon  depends,  however,  upon  the  action  or  inactivity  of 
the  cellular  tissue  itself ;  it  will  be  convenient,  for  the  better  understanding  of  it,  to  con- 
sider it  under  two  heads.  First,  where  the  tissue  exerts  no  absorbent  action  on  the 
media,  or  absorbs  both  to  the  same  extent ;  and,  secondly,  where  one  is  absorbed  to  a 
much  greater  extent  than  the  other. 

168.  In  the  first  case,  the  velocities  with  which  any  two  gases  pass  into  a  vacuum  are 
inversely  proportional  to  the  square  roots  of  their  densities  respectively ;  moreover,  the 
volumes  that  so  pass  vary  directly  as  the  velocities,  and  therefore  may  be  taken  as  an 
index  and  measure  of  them ;  but,  as  the  mass  of  each  gas  is  expressed  by  the  product 
of  its  density  into  its  volume,  it  may  be  also  represented  by  the  velocity  multiplied  into 
the  density ;  and,  as  the  square  of  the  velocity  of  the  one,  multiplied  into  its  density,  is 
equal  to  the  square  of  the  velocity  of  the  other  multiplied  into  its  density,  whatever  may 
be  the  difference  of  the  specific  gravity  of  the  two  gases,  their  mechanical  momentum 
will  always  be  the  same;  the  resistance  they  meet  with  in  passing  through  the  tissue  is 
common  to  both,  and  equal  in  both  cases  ;  and  hence  the  initial  velocities  of  diffusion 
ought  to  be  inversely  proportional  to  the  square  roots  of  the  densities ;  and  as,  during 
the  progress  of  the  experiment,  the  impelling  force  of  the  one  gas  is  equal  to  the  ex- 
pelling force  of  the  other,  the  resulting  momenta  of  the  two  currents  is  still  equal,  and 
the  final  volumes  are  such  as  are  found  by  direct  experiment. 

169.  We  now  come  to  consider  the  second  case,  where  the  cellular  tissue  presents 
one  of  the  gases  in  a  condensed  form  to  the  other,  or,  in  other  words,  absorbs  it;  and 
here  we  have  to  refer  to  a  fundamental  proposition  of  dynamics,  that  when  the  moving 
force  and  the  matter  to  be  moved  vary  in  the  same  proportion,  the  resulting  velocity 
will  always  be  the  same.  An  illustration  will  show  the  application  of  this  principle  to 
the  case  in  hand.    If  a  cylinder  of  air,  fitted  appropriately  with  a  piston,  communicates 


54 


CO-ORDINATION  OF  DALTON'S,  GRAHAM'S,  AND  MITCHELL'S  RESULTS. 


with  a  vacuum  by  means  of  an  aperture,  it  is  immaterial  whether  the  air  be  allowed  to 
flow  into  the  void  without  any  pressure,  or  whether  it  be  urged  by  a  direct  action  on 
the  piston,  its  velocity  as  it  goes  into  the  void  will  be  in  both  cases  the  same  ;  for,  if  it 
be  compressed,  the  immediate  action  of  the  force  exerted  on  the  piston  is  to  reduce  the 
air  in  the  cylinder  to  such  a  density  that  its  elasticity  shall  be  equal  to  the  compressing 
force,  and  because  the  elasticity  varies  directly  as  the  density,  the  density  of  the  air  in- 
creases with  the  impelling  force.  The  matter  to  be  moved  is  increased,  therefore,  in 
the  same  proportion  with  the  pressure,  and  therefore  the  final  velocity  is  the  same.  Now 
what  is  here  said  of  a  cylinder  of  compressed  air,  applies  evidently  to  the  action  of  a  cellu- 
lar tissue,  which  is  nothing  more  than  a  perpetual  and  equable  condensing  engine.  If  it 
increases  the  elastic  force  of  one  of  the  gases  by  compressing  it,  at  the  same  time  it  in- 
creases its  density ;  and,  therefore,  its  velocity  of  transit  is  the  same  as  though  it  had 
not  suffered  any  action  of  compression. 

170.  Such  is  the  case  while  the  gases  are  engaged  with  each  other  in  the  tissue; 
but  as  soon  as  they  are  passed  from  it,  and  are  beyond  the  reach  of  its  attractive  force, 
a  new  condition  of  things  takes  place  :  the  condensed  gas  being  no  longer  under  re- 
straint, expands  freely  into  a  void,  and  when  there  measured,  gives  a  resulting  volume 
totally  different  to  what  it  would  have  been  had  not  the  tissue  compressed  it.  Sup- 
pose, for  example,  we  placed  on  one  side  of  a  tissue  carbonic  acid,  of  which  it  would 
condense  its  own  volume,  and  on  the  other  atmospheric  air,  on  which  it  exerted  no 
action.  While  the  two  gases  were  engaged  together  in  the  tissue,  one  would  be  pre- 
sented to  the  other  under  an  elasticity  double  of  that  which  it  would  have  had  had  no 
absorption  gone  on ;  but  since  its  density  is  directly  proportional  to  its  elastic  force,  the 
continual  velocity  with  which  it  would  rush  into  the  other  gas  is  the  same  as  though 
no  compression  had  occurred ;  the  rate  of  exchange  in  the  cellular  tissue  is  the  same 
as  under  normal  circumstances  ;  that  is  to  say,  every  volume  of  air  replaces  0-8091 
of  compressed  carbonic  acid  ;  but  so  soon  as  this  gas  has  reached  the  opposite  side  of 
the  barrier,  and  there  escapes,  its  elastic  force  being  restrained  by  no  compression, 
causes  it  to  assume  its  original  dimensions. 

171.  It  will  be  readily  perceived  that  the  theory  here  given  depends  on  the  princi- 
ple, that  however  much  a  gas  is  condensed,  it  will  at  all  pressures  rush  into  a  vacuum 
with  the  same  velocity.  The  elasticity  of  a  gas  in  any  state  is  measured  by  the  force 
under  which  it  exists,  and  this  is  ordinarily  the  pressure  or  weight  of  the  atmosphere ; 
it  follows,  therefore,  that  though  the  density  of  gases  may  vary,  yet  they  have  all  the 
same  elastic  force ;  but,  when  pressure  is  exerted  upon  them,  the  density  and  elasticity 
increasing  together,  their  velocity  in  rushing  into  a  void  is  always,  and  under  all  pres- 
sures, a  constant  quantity. 

172.  We  may  now  apply  this  reasoning  to  certain  practical  cases.  Mr.  Graham 
found  that  the  absorption  of  carbonic  acid  by  a  porous  plug  of  stucco  was  very  small 
in  amount,  and  the  absorption  of  atmospheric  air  is  equally  minute.  Accordingly, 
when  these  two  gases  are  separated  from  each  other  by  a  screen  of  that  substance,  they 
diffuse  according  to  the  law  of  the  square  roots  of  their  density.  One  volume  of  air, 
replacing  0-8091  of  carbonic  acid,  the  gas,  therefore,  on  that  side  of  the  screen  where 


DISTURBING  AGENCIES,  AS  TEMPERATURE.  55 

the  carbonic  acid  was  increases  in  quantity.  Now  when,  instead  of  a  screen  of  stuc- 
co, a  thin  lamina  of  India-rubber  is  used,  which  is  found,  upon  trial,  to  condense  one 
atmosphere  of  carbonic  acid,  while  it  does  not  act  upon  air,  the  same  rate  of  exchange 
ensues ;  but  there  is  a  diminution  of  gaseous  matter  on  the  side  containing  the  acid, 
and  because  the  screen  condenses  one  atmosphere,  there  should  be  found  only  half  as 
much  gas  as  would  represent  the  equivalent  volume  of  diffusion  had  the  screen  pos- 
sessed no  condensing  power. 

173.  One  hundred  and  sixty-one  measures  of  carbonic  acid  gas  were  confined  in  a 
tube  under  a  thin  sheet  of  India-rubber,  and  suffered  to  diffuse  for  thirty-six  hours.  To 
prevent  as  much  as  possible  any  disturbing  action  of  the  fluid  over  which  the  experi- 
ment was  tried,  a  saturated  solution  of  common  salt,  which  absorbs  carbonic  acid  slow- 
ly, was  made  use  of  The  gaseous  contents  of  the  tube  decreased  in  their  dimensions 
very  rapidly,  and  when  measured,  were  found  to  consist  of  98  volumes  only.  In  the 
mean  time,  a  tube  closed  at  one  end,  filled  with  the  same  quantity  of  carbonic  acid,  and 
placed  by  the  side  of  the  former,  had  decreased  about  five  measures ;  we  may  therefore 
assume  that  the  quantity  of  gas  that  should  have  been  found  in  the  diff'nsion  tube  ought 
to  have  amounted  to  100  measures  nearly.  Now  the  specific  gravity  of  carbonic  acid 
gas  is  1-527,  the  reciprocal  of  the  square  root  of  which  is  0-8091.  Hence,  under  ordi- 
nary circumstances,  one  volume  of  air  should  replace  0-8091  of  carbonic  acid  gas;  but 
as,  in  the  experiment  here  tried,  the  barrier  produced  a  compression,  one  volume  of  air 
should  displace  1-6182  of  carbonic  acid,  the  amount  observed  very  nearly. 

]  71.  I  would  not  here  be  understood  to  say  that  there  are  no  other  disturbing  ac- 
tions going  on  in  cellular  tissues  except  those  which  result  from  their  absorbent  power. 
A  great  many  facts  show  that,  under  peculiar  conditions,  they  are  able  to  produce  de- 
compositions of  a  certain  sort.  Often  their  regular  action,  as  indicated  by  theory,  seems 
to  be  entirely  departed  from  ;  great  disturbance  arising  from  the  fact,  that  when  two  gases 
are  absorbed  together  by  any  areolar  tissue,  they  experience  a  greater  condensation  than 
each  would  in  a  separate  state.  The  presence  of  nitrogen  or  carbonic  acid,  in  any 
porous  mass,  increases  the  action  of  that  mass  on  oxygen,  more  of  the  latter  being  con- 
densed. A  piece  of  charcoal,  impregnated  with  oxygen,  condenses  more  hydrogen  than 
it  should  do,  and  the  presence  of  hydrogen  facilitates  the  condensation  of  nitrogen.  It 
is,  therefore,  impossible  to  foretell  what  the  result  of  diffusing  one  gas  into  another  will 
be,  by  simply  ascertaining  how  many  volumes  of  either  alone  will  be  absorbed  by  the 
tissue,  inasmuch  as  a  greater  or  lesser  condensation  may  happen  when  both  are  em- 
ployed together. 

175.  Variations  of  temperature,  which  probably  affect  the  power  of  absorption,  and 
thereby  the  diff'nsion  of  volumes,  are  experienced  by  all  tissues.  When  charcoal,  or 
any  other  porous  mass,  is  placed  in  an  atmosphere  of  gas,  which  it  can  condense  rap- 
idly, its  temperature  rises,  the  effect  apparently  depending  more  on  the  velocity  of  ab- 
sorption than  on  the  final  amount.  In  the  case  of  ammonia,  it  does  not  even  require  a 
thermometer  to  discover  this  increase  of  temperature,  for  it  is  very  sensible  to  the  touch. 
On  the  other  hand,  when  this  condensed  gas  makes  its  escape,  a  corresponding  dimi- 
nution of  temperature  happens  :  it  is  immaterial  by  what  means  the  liberation  of  the  gas 


6^ 


PHYSIOLOGICAL  EXPERIMENTS  AND  REMARKS. 


is  effected,  the  same  result  uniformly  follows  ;  if,  for  example,  a  porous  mass  saturated 
with  carbonic  acid  be  exposed  to  an  air-pump  vacuum,  in  connexion  with  a  thermo- 
metric  arrangement,  the  gas,  as  it  is  liberated  from  the  pores  of  the  structure  by  the  ac- 
tion of  the  pneumatic  machine,  gives  rise,  by  its  expansion,  to  the  production  of  cold. 
Or,  if  the  same  porous  mass,  saturated  in  like  manner  with  carbonic  acid,  be  exposed  to 
an  atmosphere  of  hydrogen,  it  absorbs  but  a  small  quantity  of  this  latter  substance, 
while  a  very  large  amount  of  the  former  is  liberated  from  its  condensed  state,  and  the 
thermometer  indicates  a  fall  of  temperature  ;  the  resulting  volume  of  the  mixed  gases 
being  much  larger  than  the  original  volume  of  hydrogen.  Again,  if  a  porous  mass, 
which  has  absorbed  its  due  volume  of  hydrogen,  be  immersed  in  an  atmosphere  of  am- 
monia, the  resulting  volume  of  the  mixed  gases  is  much  smaller  than  the  original  amount, 
and  the  porous  mass  becomes  hot. 

176.  The  observations  here  made  on  the  vicissitudes  of  temperature  which  an  areo- 
lar mass  experiences,  when  successively  immersed  in  an  atmosphere  of  different  kinds, 
obviously  apply  when  the  exposures,  instead  of  being  consecutive,  are  simultaneous.  If, 
for  example,  a  barrier  separates  carbonic  acid  and  hydrogen  gas,  and  absorbs  the  former 
to  a  large  amount,  but  exerts  little  or  no  action  on  the  latter,  then  the  opposite  sides  of 
that  barrier  will  be  unequally  heated.  Suppose,  for  illustration,  we  call  that  surface  of 
the  barrier  which  looks  towards  the  carbonic  acid,  C,  and  the  surface  looking  towards 
the  hydrogen,  H:  then,  because  of  the  condensing  action  of  the  barrier  on  the  acid  gas, 
the  surface  C  will  become  hot ;  but  because  the  gas,  so  soon  as  it  has  passed  the  barrier, 
expands,  as  into  a  void,  when  it  reaches  the  surface  H  that  surface  will  become  cold. 
We  see,  therefore,  that  immediately  after  the  action  of  the  membrane  or  barrier  is  first 
set  up,  the  absorption  of  the  carbonic  acid  takes  place  on  a  hot  surface,  and  its  evolu- 
tion from  a  cold  one ;  whereas  the  absorption  of  the  hydrogen  takes  place  on  a  cold 
surface,  and  its  liberation  from  a  hot  one.  A  modified  result  of  course  happens  when 
both  gases  are  absorbed  in  different  degrees,  and  any  prediction  of  the  resulting  action 
becomes  a  matter  of  much  difficulty.  Where  the  barrier  is  very  thin,  or  has  a  high  con- 
ducting power  as  respects  caloric,  this  distinct  surface  action  may  not  rigidly  occur,  but 
the  whole  of  the  structure  experiences  some  determinate  rise  or  diminution ;  a  mean  of 
the  condition  of  the  two  surfaces  respectively, 

177.  The  obvious  application  of  these  results,  in  a  physiological  point  of  view,  is  to 
the  function  of  respiration.  In  no  order  of  life,  however,  does  the  respiratory  mechanism 
coincide  with  the  arrangements  of  two  gaseous  media  separated  from  each  other  by  a 
barrier.  In  those  tribes  vi^hich  breathe  by  lungs,  the  pulmonary  vessels  present  themselves 
on  the  remote  bronchial  cells,  and  the  arrangement  is,  in  effect,  a  liquid  and  a  gas,  parted 
from  each  other  by  a  membrane.  Chemical  physiologists  have  hastened  to  apply  the 
discovery  of  Dr.  Mitchell  in  this  case,  and  have  done  right.  But  still  the  chain  of 
evidence  is  incomplete,  for  we  have  not  yet  seen  it  proved  that  gaseous  matter,  in  union 
with  a  liquid,  will  leave  it  and  pass  through  a  barrier  to  join  a  gas  on  the  other  side. 
The  following  experiment  will  supply  this  defect :  A  small  jar,  the  mouth  of  which 
was  closed  with  India-rubber,  and  its  opposite  end  made  to  terminate  a  tube  one  eighth 
of  an  inch  in  diameter,  while  full  of  atinospheric  air,  was  sunk  in  a  vessel  containing 


RELATION  OF  ENDOSMOSIS  TO  CAPILLARY  ATTRACTION.  57 

water  impregnated  with  carbonic  acid :  in  a  very  sliort  time,  the  acid  gas,  leaving  the 
water,  went  through  the  barrier,  and  as  it  accumulated  in  the  jar,  was  delivered  hy  the 
short  tube  at  the  other  end,  and  passed  up  in  bubbles  through  the  water. 

178.  Branchial  respiration  deviates  still  more  from  the  simple  type,  for  we  have  here 
two  fluids,  presenting  gaseous  matter  to  each  other  for  interchange  through  a  membra- 
nous screen.  In  one  of  them  the  gas  is  in  a  state  of  solution  only,  but  as  to  what  its 
condition  in  the  other  may  be,  we  can  scarcely  say.  The  phenomenon,  however,  becomes 
obviously  much  more  complex.  In  bronchial  respiration,  the  account  which  Mr.  Graham 
gives  of  the  process  by  which  the  little  cells  empiy  themselves  into  the  trachea  is  prob- 
ably correct ;  and  the  same  observation  undoubtedly  applies  to  the  case  of  the  respiration 
of  insects. 

179.  The  issue  of  these  investigations,  besides  co-ordinating  the  observations  of  Dr. 
Mitchell  and  Professor  Graham,  has  a  far  more  important  application.  It  shows  us 
indisputably  that  membranes  have  special  mechanical  functions  depending  on  the  condi- 
tions of  their  texture ;  and  that  often  they  are,  in  appearance,  the  generators  of  power 
equal  to  the  pressure  of  many  atmospheres.  It  is  not  pretended,  however,  that  the 
foregoing  paragraphs  contain  the  whole  theory  of  cellular  action,  the  object  of  this 
communication  being  limited  to  a  discussion  of  some  of  those  mechanical  functions 
which  have  led  chemists  to  conflicting  results.  Writers  on  physiology  have  suspected 
that  membranes  were  springs  of  power,  both  mechanical  and  chemical,  but  the  direct 
proof,  from  actual  experiment,  has  never  until  now  been  furnished. 


CHAPTER  VII. 

THE   PHYSICAL  THEORY  OF  ENDOSMOSIS, 

{From  the  American  Journal  of  the  Medical  Sciences  for  August,  1838.) 

Contents  :  Relation  of  Endosmosis  to  Capillary  Attraction. — Cases  of  reported  Decom- 
positions.— Can  he  produced  hy  Inorganic  Masses,  and  therefore  not  due  to  Vitality. — 
•.Water  ?}iade  to  wet  Mercury. —  Voltaic  Battery  controls  Capillary  Attraction. — Ac- 
tion of  Inorganic  Tissues. —  Water  passes  through  excessively  small  Pores. — Hy- 
draulic Currents. — Deposites  produced  hy  Endosmotic  Currents. — Apparent  Decompo- 
sition of  Metallic  Salts  hy  Memhranes. —  True  Theory  of  it. — General  Conclusion  that 
Endosmosis  is  VjOtMng  more  than  common  Capillary  Attraction,  and  never  occasions 
true  Decompositions. 

180.  It  is  the  object  of  this  communication  to  offer  some  proofs  that  the  peculiar 
force  known  to  chemists  and  physiologists  under  the  title  of  endosmose  and  exosmose, 
has  no  existence  independently  of  ordinary  capillary  attraction ;  and  that  all  the  cases 
described  as  chemical  decompositions,  brought  about  by  the  intervention  of  animal 
membranes  and  areolar  tissues,  are  only  examples  of  the  play  of  ordinary  and  well- 
known  agents. 

H 


58 


RELATION  OF  ENDOSMOSIS  TO  CAPILLARY  ATTRACTION. 


181.  It  is  necessary,  before  entering  into  a  critical  examination  of  tliese  points,  to 
explain  briefly  the  leading  experiments  which  have  been  reported  in  connexion  with 
the  subject.  Often  received  upon  doubtful  evidence,  and  sometimes  implying  the  exist- 
ence of  laws  wliich,  if  established,  would  compel  us  to  modify  our  opinions  of  chemical 
agency  in  general,  it  is  time  that  the  whole  of  them  should  pass  under  a  connected 
review,  their  bearings  upon  each  other  be  properly  designated,  and  the  true  value  which 
ought  to  be  attached  to  them  ascertained.  If  thus  examined,  it  will  be  found  that  they 
are  very  far  from  establishing  the  points  supposed ;  and  in  compliance  with  the  usages 
of  science,  their  application  must  be  rejected. 

182.  We  shall  have  to  consider,  1st,  those  experiments  which  refer  to  changes  of 
hydrostatic  level  of  liquids,  and  the  production  of  mechanical  results;  2dly,  those  which 
are  reputed  to  be  examples  of  chemical  depositions  brought  about  by  tissue  action. 

83.  The  original  experiments  of  Porrett,  Fischer,  and  Dutrochet,  are  instances  of 
the  first  class ;  from  them  the  terms  endosmose  and  exosmose  are  derived.  They  are 
essentially  illustrations  of  the  fact,  that  if  two  fluids  be  separated  from  one  another  by 
a  porous  harrier,  they  will  mutually  traverse  it,  but  very  often  not  with  equal  velocities, 
for  the  volume  of  the  one  passing  in  a  given  time  may  exceed  the  volume  of  the  other, 
and  hence  a  disturbance  of  their  hydrostatic  level  results.  If,  for  example,  we  take  a 
tube,  and  close  one  of  its  ends  with  a  piece  of  bladder,  securely  tied  on,  and  fill  it  to  a 
certain  mark  with  alcohol,  and  then  place  it  in  a  vessel  of  water,  taking  care  that  the 
hydrostatic  level  inside  the  tube,  and  that  on  the  outside,  shall  coincide,  in  the  course 
of  a  few  hours  it  will  be  found  that  this  equilibrium  has  been  entirely  disturbed,  and  the 
level  of  the  alcohol  risen.  On  reversing  the  arrangement,  and  placing  the  water  in 
the  tube  and  alcohol  on  the  outside,  there  will  be,  at  the  completion  of  the  experiment, 
a  similar  disturbance,  but  now  the  level  will  be  found  to  have  fallen. 

"  (r/.)  In  this  way  it  was  found  that  there  was  endosmose  from  water  to  gum-water, 
to  acetic  acid,  to  nitric  acid,  and  especially  to  hydrochloric  acid  ;  but  that  there  was 
not  endosmose  from  a  liquid  to  itself. 

"  (b.)  And  that  different  animal  and  vegetable  membranes  enjoyed  the  same  proper- 
ties as  bladder,  in  different  degrees,  and  that  plates  of  burned  earth,  or  calcined  slate,  or 
clay,  and,  in  general,  all  aluminous  substances,  possessed  analogous  powers,  though  to 
a  much  less  extent. 

"  (c.)  To  explain  these  phenomena,  it  is  necessary  to  resort  to  some  force  different 
from  ordinary  capillary  attraction,  or,  at  least,  to  some  new  modification  of  it ;  for  the 
forces  of  capillarity,  such  as  they  are  now  understood,  are  totally  insufficient  to  produce 
these  results." — (Poui/ief.) 

184.  The  second  class  of  experiments,  though  often  affording  well-marked  illustra- 
tions of  change  of  hydrostatic  level,  is  chiefly  important  from  the  instances  of  apparent 
decomposition  exhibited.  From  these  it  has  been  inferred,  not  only  that  membranes 
possessed  certain  definite  chemical  powers,  but  that  at  times  they  gave  proof  of  a  predi- 
lection for  the  passage  of  certain  bodies  in  determinate  directions  through  them. 

185.  If  litmus  water  be  placed  on  one  side  of  a  piece  of  bladder,  and  alcohol  on  the 
other,  the  water  will  forsake  the  colouring  matter  to  pass  through  the  bladder  and  unite 
with  the  alcohol. 


CAN  BE  PRODUCED  BY  INORGANIC  MASSES.  59 

186.  If  ferrocyanate  of  potassa  be  tied  up  in  a  section  of  intestine,  and  immersed  in  a 
solution  of  protosulphate  of  iron,  Prussian  blue  will  be  deposited  on  one  side  of  the 
intestine,  but  not  on  the  other ;  hence  it  is  inferred  that  one  solution  is  suffered  to  pass 
through  the  pores,  but  a  like  passage  is  denied  to  the  other. 

187.  If  a  solution  of  oxalic  acid  be  placed  on  one  side  of  a  membrane,  and  lime- 
water  on  the  other,  clouds  of  the  insoluble  oxalate  of  lime  will  form  on  the  side  of  the 
lime-water,  but  the  other  side  will  be  pellucid. 

188.  If  a  volume  of  nitrogen  gas  in  a  soap-bubble,  or  under  any  suitable  membrane, 
be  exposed  to  atmospheric  air,  decomposition  of  that  air  will  result,  its  oxygen  passing 
through  the  membrane  to  form  atmospheric  air  with  the  nitrogen  within. 

189.  If  a  quantity  of  commercial  alcohol  be  tied  up  in  a  bladder,  and  freely  exposed 
to  the  air,  the  water  in  union  with  that  alcohol  will  pass  through  the  pores  of  the  blad- 
der, and,  gradually  evaporating  away,  will  leave  the  alcohol  nmch  stronger. 

190.  And,  lastly,  which  is  by  far  the  most  remarkable  of  these  phenomena,  if  a  tube, 
the  extremity  of  which  is  closed  with  membrane,  be  filled  to  a  certain  height  with 
distilled  water,  and  there  be  placed  in  it  a  few  iron  nails,  on  adjusting  it  hydrostatically, 
and  suffering  it  to  remain  for  a  time  in  a  solution  of  sulphate  of  copper,  the  membrane 
will  apparently  decompose  the  solution  of  the  metallic  salt,  the  base  of  which,  in  a 
deoxidized  state,  will  remain  attached  to  the  under  side  of  the  membrane,  but  the  acid 
and  oxygen  will  traverse  it,  and  be  removed  by  uniting  with  the  iron. 

191.  The  body  of  evidence  here  furnished  would  go  to  show  that  membranes  possess 
remarkable  habitudes  with  respect  to  liquids,  and,  accordingly,  it  has  been  brought 
forward  as  the  foundation  of  many  physiological  hypotheses.  Nay,  more,  from  hence  it 
has  been  assumed  that  these  were  in  truth  nothing  more  than  manifestations  of  that 
principle  of  vitality  which  is  supposed  to  be  the  result  of  organization.  A  power, 
known  under  the  name  of  endosmose,  distinct  from  all  other  known  agents,  has  been 
created,  its  especial  office  being  to  bring  about  certain  molecular  changes,  in  a  way  re- 
sembling outwardly,  but  essentially  differing,  from  those  of  chemical  affinity. 

192.  The  error  of  this  position  might  readily  have  been  detected.  We  surely  should 
not  regard  that  as  a  specific  force  of  vitality  which  is  possessed  by  inorganic  matter; 
yet,  in  the  outset  of  the  original  experiments  on  the  subject,  it  was  found  that  alumina 
exhibited  the  same  action  as  bladder,  though  in  a  feebler  degree.  As  to  the  amount  of 
force,  with  that  we  have  nothing  to  do  ;  for,  no  matter  in  how  small  a  degree  soever  it  may 
be  that  alumina  possesses  this  character,  the  mere  fact  of  possessing  it  at  all  goes  to  show 
that  it  is  not  a  consequence  of  organization,  or  an  evidence  that  the  substance  exhibiting 
it  has  ever  been  moulded  by  the  powers  of  life. 

193.  The  verification  of  Dutrochet's  experiment  with  alumina  becomes,  therefore,  a 
matter  of  the  greatest  importance  ;  its  extension  to  other  inorganic  substances  would  de- 
cide the  point,  and  separate  at  once  the  power  by  which  infiltrations  take  place  from 
the  powers  of  vitality.  It  has,  however,  been  stated  that  those  minerals  in  which  this 
property  has  been  observed  possess  it  in  a  low  degree.  Some  chemists  have  extended 
this  observation,  and  class  with  alumina  other  bodies  of  a  porous  texture,  as  certain  varie- 
ties of  slate.    But  many  experiments  that  have  been  made  on  this  point  have  led  to  er- 


60 


IS  THEREFORE  NOT  DUE  TO  VITALITY. 


roneous  results,  through  inattention  to  the  conditions  of  iiydrostatic  equihbrium.  If 
two  fluids  be  placed  in  the  opposite  arms  of  an  inverted  siphon,  they  will  have  a  com- 
mon level  only  when  their  specific  gravity  coincides  ;  and,  under  all  other  circumstances, 
the  height  of  their  columns  will  be  inversely  proportional  to  their  specific  gravity  re- 
spectively. If,  then,  we  take  a  tube,  and  make  its  extremity  end  in  a  fine  capillary  termi- 
nation, or  close  it  with  a  plug  of  wood  or  of  stucco,  and  fill  it  with  some  dense  solution, 
such  as  chloride  of  sodium,  sulphate  of  potash,  or  sulphate  of  copper,  and  then  immerse 
it  to  the  same  level  in  pure  water,  the  level  of  the  fluid  in  the  tube  will  descend  in 
obedience  to  the  laws  of  hydrostatics,  and  when  a  position  of  equilibrium  is  gained,  the 
heights  of  the  fluid  inside  and  outside  of  the  tube  will  be  inversely  proportional  to  their 
specific  gravities.    In  all  this,  endosmosis  or  capillary  attraction  has  no  concern. 

194.  In  structures  the  pores  of  which  are  of  such  a  diameter  that  these  adjustments  of 
level  can  freely  take  place,  the  mechanical  phenomena  of  endosmose  are  not  visible ; 
there  is  no  fact  that  can  indicate  what  is  the  true  action  of  the  porous  body.  All 
bodies  which  exhibit  these  phenomena  have  their  pores  of  such  a  size  that,  while  they 
offiir  resistance  to  change  of  level  by  mere  leakage,  they  allow  indefinitely  small  colums  of 
the  fluid  they  are  exposed  to  to  interchange  through  them. 

195.  We  are  not  to  expect  that  any  of  the  phenomena  of  molecular  infiltration  will  be 
exhibited  when  the  apertures  through  which  transudation  occurs  are  of  considerable  size. 
If  a  piece  of  coarse  linen  is  made  use  of  as  the  separator  of  two  fluids,  those  fluids  will 
commingle  without  any  disturbance  of  hydrostatic  level.  Whenever  this  latter  equilibrium 
can  be  eflected,  it  takes  place,  and  entirely  masks  the  molecular  action  of  the  mass.  For 
this  reason,  most  minerals  fail  to  show  the  change  of  level  when  water  passes  into  alcohol. 
They  do  not  possess  the  exact  kind  of  porosity  required,  either  having  their  interstices 
of  so  large  a  size  that  derangements  of  level  can  be  quickly  compensated,  or,  on  the 
other  hand,  being  totally  impervious  to  the  liquids.  I  found  that  the  common  white 
earthenware,  when  its  glaze  was  removed,  allowed  water  to  percolate  through  it  to  gum- 
water,  but  no  disturbance  of  level  was  observed,  simply  because  the  freedom  of  commu- 
nication between  the  two  hquids  was  so  great,  that  if  one  of  them  had  a  higher  level  than 
the  other  purposely  given  to  it,  it  soon  returned  to  its  original  position  of  equilibrium. 
A  fragment  of  thick  Hessian  crucible  gave  the  same  result,  as  also  several  varieties  of 
slate,  iron  slate  and  mica  slate,  some  of  which  were  calcined  and  others  in  their  ordi- 
nary condition ;  also  a  fragment  of  common  writing  slate,  which  had  undergone  semi- 
vitrification  in  the  forge.  This  freedom  of  comnumi('ation  was  noticed  in  some  speci- 
mens of  soapstone,  both  burned  and  unburned ;  and  while,  in  plaster  of  Paris,  the  ex- 
periment failed  because  the  apertures  were  too  large,  in  the  transparent  micas  it  failed 
for  want  of  communication.  To  show,  therefore,  the  original  experiment  of  Dutrochet, 
the  interstices  of  the  barrier  must  be  of  such  a  diameter  that  all  mechanical  compensa- 
tions for  change  of  level  arc  hindered,  and  free  molecular  infiltration  can  take  place. 

19(5.  If  a  tube  half  an  inch  in  diameter  and  three  or  four  inches  long  be  sealed  at  one 
end,  and  while  the  glass  is  yet  warm  be  dipped  into  water,  a  number  of  small  cracks 
will  be  made  in  its  bottom.  This  forms  a  very  useful  instrument  for  studying  the 
properties  here  under  discussion.    If  it  be  filled  to  a  certain  mark  with  alcohol,  and 


WATER  MADE  TO  WET  MERCURY.  Qi 

then  plunged  to  the  same  level  in  water,  an  apparent  endosmosis  through  the  cracks  is 
the  result,  for  the  alcohol  rises  with  considerable  velocity  in  the  tube.  It  is,  however, 
only  an  apparent  endosmosis,  for,  upon  closer  examination,  it  will  be  found  that  the 
motion  stops  when  the  hydrostatic  equilibrium  is  adjusted  through  the  chink  (193).  Any 
of  the  very  porous  minerals  show  the  same  thing. 

197.  If,  in  the  last  experiment,  the  tube  be  filled  with  lime-water,  and  then  immersed 
to  the  same  level  in  a  solution  of  oxalic  acid,  the  appearance  described  in  (187)  will  be 
reproduced.  This  cautions  us  not  to  impute  to  membranes  any  predilection  for  passage 
in  certain  directions  ;  for  that  may  arise  from  extraneous  circumstances,  and  in  the  in- 
stance referred  to,  as  will  presently  be  shown,  originates  in  a  very  diiferent  cause. 

198.  The  relation  existing  between  solids  and  fluids  which  determines  their  descent 
or  rise  in  capillary  tubes,  has  been  referred  to  heretofore  in  these  papers  (123).  A  con- 
nexion of  the  phenomena  of  endosmosis  and  capillary  attraction  might  have  been  traced  to 
the  fact  that  no  liquid  will  pass  through  a  barrier  the  surface  of  the  pores  of  which  it 
cannot  wet.  The  relation  of  glass  and  quicksilver  to  each  other,  in  this  point  of  view,  is 
interesting.  When  a  piece  of  glass  is  laid  upon  the  surface  of  this  fluid  metal,  contact 
between  them  does  not  take  place,  but  they  are  separated  from  each  other  by  an  ex- 
ceedingly small  interval.  As  I  had  failed  in  reproducing  many  of  the  results  attempted 
by  means  of  artificial  chinks  in  glass  (196),  because  of  their  magnitude,  I  was  led  to 
hope  that  better  success  would  attend  the  same  attempts  by  making  use  of  the  small 
interstice  between  glass  and  mercury.  A  tube  half  an  inch  in  diameter  was  therefore 
taken,  and  one  of  its  extremities,  having  been  ground  truly  flat,  had  its  roughness  taken 
off  by  exposing  it  carefully  to  the  blowpipe  flame.  When  the  tube  was  lowered  with 
this  extremity  downward,  on  the  surface  of  some  pure  mercury,  all  the  parts  of  its 
circumference  touched  the  metal  at  once.  A  solution  of  green  vitriol  was  placed  in  it 
to  a  certain  height,  and  upon  the  mercury  ;  on  the  outside  of  it  was  poured  a  solution  of 
ferrocyanate  of  potassa.  It  was  expected  that,  through  the  chink  between  the  mercury 
and  the  glass,  the  liquids  would  slowly  infiltrate  to  each  other.  After  several  days,  no 
such  action  was  observed,  and  the  experiment,  though  repeated  under  a  variety  of  con- 
ditions, afforded  no  better  result.  Now  water  and  saline  solutions,  as  will  hereafter  be 
shown,  pass  through  interstices  much  more  minute  than  this  can  reasonably  be  supposed 
to  be  ;  it  is  evident,  therefore,  that  the  want  of  action  is  mainly  due  to  the  circumstance 
that  water  and  saline  solutions  generally  do  noi  wet  mercury,  and  the  laws  of  capillary 
action  would  indicate  that,  under  these  circumstances,  they  would  fail  to  pass  through 
the  chink. 

199.  The  event  of  this  experiment  points  out,  in  an  impressive  manner,  the  general 
relations  that  must  exist  between  the  solids  and  liquids  of  organized  bodies.  Water 
will  pass  with  great  rapidity  through  a  chink  the  width  of  which  is  not  more  than  the 
half  of  a  millionth  part  of  an  inch,  provided  it  can  wet  both  sides  of  that  chink  ;  but  if 
this  condition  be  not  fulfilled  it  fails  to  pass,  even  though  the  width  should  increase  to 
upward  of  one  hundred  and  forty-four  times  its  former  dimensions. 

200.  That  the  non-passage  described  is  here  referred  to  the  true  cause,  will  appear 
from  the  following  experiments.    As  has  been  stated,  under  ordinary  circumstances, 


62 


VOLTAIC  BATTERY  CONTROLS  CAPILLARY  ATTRACTION. 


water  does  not  wet  the  surface  of  pure  quicksilver,  but  stands  upon  it  in  drops  of  a 
more  or  less  rounded  form  :  if,  however,  the  electrical  relation  of  these  substances  be 
changed — if  the  mercury  be  in  contact  with  the  negative  pole  of  a  voltaic  battery,  and 
the  water  with  the  positive,  a  remarkable  phenomenon  ensues — the  water  now  wets  the 
mercury.    To  this  important  fact,  and  its  applications,  I  shall  hereafter  return. 

201.  The  best  method  of  showing  that  the  voltaic  battery  has  entire  control  over 
capillary  attraction  is  to  take  a  shallow  vessel  containing  a  quantity  of  mercury,  as  A  A, 
iig.  23,  and  place  upon  it,  in  the  position  marked  <?,  a  drop  of  water  ;  on  making  this 
drop  comnmnicate  with  the  positive  electrode  of  a  battery,  and  the  mercury  with  the 
negative,  in  a  moment  the  drop  loses  its  rounded  form  and  spreads  out  in  a  thin  sheet 
on  the  metallic  surface,  completely  wetting  it,  and  according  as  the  tension  of  the  bat- 
tery increases,  the  drop  expands  more  and  more.  Thus,  if  the  current  from  5,  10,  20, 
40,  80,  &c.,  plates  be  successively  passed  through  it,  the  diameter  of  the  circular  space 
it  occupies  closely  follows  the  increase,  and  appears  to  continue  to  do  so  until  the  drop 
becomes  so  thin  that  the  electricity,  in  the  shape  of  a  spark,  can  pass  through  :  then, 
of  course,  the  experiment  cannot  be  continued. 

202.  If,  therefore,  in  the  arrangement  of  (198),  the  electrical  relations  of  the  saline 
solution  and  the  mercury  be  changed,  by  the  process  here  indicated,  we  should  expect 
that  the  passage  through  the  chink  would  take  place  ;  and  it  is  so.  This  experiment 
affords  a  very  elegant  illustration  of  a  result  obtained  by  Porrett  many  years  ago,  which 
was  applied  by  Dutrochet  to  the  explanation  of  endosmose.  He  observed  that  if  two 
quantities  of  water  were  separated  from  each  other  by  a  membranous  partition,  and 
one  of  them  made  positive  and  the  other  negative,  all  the  water  in  contact  with  the 
positive  pole  would  escape  through  the  membrane  into  this  negative  partition.  When, 
in  the  arrangement  of  (198),  the  water  escapes  through  the  chink  on  being  electrified, 
it  does  not  move  slowly,  by  the  action  of  its  own  weight,  but  is  also  impelled  downward 
in  the  way  described  by  Porrett.  For,  take  a  tube,  c  c,Jig.  24,  the  diameter  of  which  is 
about  one  tenth  or  one  twelfth  of  an  inch,  and  insert  in  its  axis  a  platina  wire,  a,  then 
i;  t  the  lower  extremity  touch  a  surface  of  water,  and  a  volume  of  that  fluid  will  rise  in 
it  to  a  certain  height  by  common  capillary  attraction.  If,  now,  the  tube,  charged  with 
its  water  and  wire,  be  placed,  as  in  the  figure,  on  the  surface  of  some  mercury  in  a 
watch-glass,  so  that  the  extremity  of  the  tube  shall  just  touch  the  metallic  surface,  and 
the  wire  a  be  then  connected  with  the  positive  electrode,  and  the  wire  h  with  the 
negative,  in  an  instant  the  water  will  begin  to  flow  out  of  the  tube,  and  spread  over  the 
mercury,  and  will  continue  to  do  so  until  its  level  has  sunk  to  the  end  of  the  platina 
wire.  With  a  wider  tube,  such  as  that  described  in  (198),  this  passage  might  be  impu- 
ted to  the  mere  gravitation  of  the  parts  of  the  fluid  urging  them  downward  ;  but,  in  this 
instance,  owing  to  the  narrowness  of  the  tube,  that  force  is  nullified  by  capillary  attrac- 
tion. The  water  is  therefore  driven  out  of  the  tube  by  an  active  force ;  and  that  this 
is  really  the  case,  is  abundantly  proved  by  breaking  the  battery  connexion  and  rais- 
ing the  tube  slightly  above  the  mercurial  surface ;  the  water  then  precipitately  re- 
turns back  into  the  tube  again. 

203.  In  Chapter  V.  it  was  shown  how  the  common  phenomena  of  capillary  attrac- 


ACTION  OF  INORGANIC  TISSUES.  g3 

tion  originate  in  electrical  excitement.  The  fact  that  electricity  has,  therefore,  an  en  - 
tire control  over  the  motions  of  fluids  in  capillary  tubes,  will  not  be  at  all  surprising. 
This  leads  us  to  a  generic  resemblance  between  the  phenomena  of  chemical  affinity  and 
those  of  capillarity,  which  deserves  a  much  more  detailed  investigation.  Treatises  on 
chemistry  represent  a  number  of  disturbing  agencies  which  frequently  antagonize,  and 
often  control  the  operations  of  affinity  ;  these  are  cohesion,  elasticity,  quantity  of  mat- 
ter, gravity,  and  the  agency  of  the  imponderables.  It  is,  however,  a  mistake  to  enumer- 
ate either  quantity  of  matter  or  gravity  as  ever  disturbing  the  action  of  affinity.  Grav- 
ity, it  is  true,  may  cause  the  lower  parts  of  a  solution  or  an  alloy  to  be  denser  than  the 
upper;  but  an  action  of  this  kind  is  not  to  be  accounted  as  an  example  of  contrariety 
in  the  forces.  It  does  not  exhibit  them  at  all  at  variance  with  each  other,  or  in  any 
manner  neutralizing  each  other.  Similar  remarks  might  be  made  in  respect  to  quantity 
of  matter.  The  elastic  state,  being  merely  a  condition  of  cohesion,  influences  the  ac- 
tion of  affinity  by  presenting  bodies  under  a  modified  form  as  respects  their  cohesion. 
Strictly  speaking,  there  are  but  two  forces  which  in  reality  control  affinity  ;  these  are 
cohesion  and  the  agency  of  the  imponderables.  And  these  are  the  forces  that  control 
capillary  action,  the  phenomena  of  which  are  the  results  of  the  equilibrium  of  an  attrac- 
tive force  on  the  one  hand,  and  cohesion  on  the  other.  They  may  be  regarded  as 
modified  cases  of  chemical  affinity,  and,  being  brought  about  by  the  operation  of  the 
same  forces,  are  under  the  control  of  the  same  disturbing;  agents. 

204.  In  (195)  I  have  enumerated  different  cases  of  ineffectual  attempts  to  recognise 
the  action  of  endosmosis  in  inorganic  bodies.  I  have  also  shown  the  peculiar  disturbance 
that  arises  when  interstitial  communication  is  too  free,  and  the  relation  that  must  exist 
between  a  solid  and  a  fluid  for  molecular  transudation  to  happen.  Now,  when  all  these 
conditions  are  fulfilled  in  any  barrier,  the  phenomena  of  endosmosis  will  take  place  ir- 
respective of  its  nature,  whether  it  be  organic  or  inorganic.  Plates  of  kaolin  or  porce- 
lain clay  from  Villarica,  disks  of  steatite  from  Brazil,  after  undergoing  induration  in  the 
fire,  and  a  variety  of  compact  sandstone  being  cemented  on  the  end  of  a  tube,  exhibited 
in  a  very  satisfactory  manner  the  passage  of  water  into  gum-water,  even  against  hy- 
drostatic pressures  of  several  inches.  Independent  of  this  decisive  evidence,  it  might 
be  determined  that  an  organized  tissue  is  not  essential  to  this  process,  from  the  circum- 
stance that  common  writing  paper,  fastened  with  sealing  wax  on  the  end  of  an  open 
tube,  exhibits  the  endosmosis  of  water  into  gum-water  in  a  much  more  striking  manner 
than  bladder ;  and  certain  inspissated  juices  of  plants,  as  caoutchouc,  when  in  thin  lay- 
ers, act  very  well,  though  not  so  rapidly.  Of  all  substances  hitherto  tried,  filtering  pa- 
per, imbued  with  coagulated  albumen,  acts  most  satisfactorily. 

205.  A  repetition  of  these  experiments,  made  under  a  variety  of  circumstances,  leaves 
no  larther  doubt  as  to  the  true  character  of  Dutrochet's  endosmosis.  It  is  not,  as  some 
would  have  us  suppose,  a  vital  or  a  semi-vital  force ;  it  is  nothing  more  than  a  peculiar 
case  of  capillary  action. 

206.  The  conditions  which  this  peculiar  case  requires  are,  that  both  the  fluids  shall 
be  able  to  wet  the  barrier,  that  in  a  capillary  tube  formed  of  it  they  should  rise  to  dif- 
ferent heights,  and  that  they  should  be  able  to  unite  chemically  with  each  other.    If  we 


€4 


WATER  PASSES  THROUGH  EXCESSIVELY  SMALL  PORES. 


suppose  a  tube  of  such  a  length,  with  respect  to  its  diameter,  that  a  fluid  in  which  it  is 
immersed  shall  rise  to  the  top  of  it,  and  that  some  extraneous  cause  shall  effect  its  re- 
moval as  fast  as  it  reaches  that  position,  it  is  evident  that  a  continuous  current  will 
traverse  the  tube.  A  case  in  point  is  the  action  of  the  wick,  of  a  lamp,  along  which 
the  oil  continually  ascends,  because  it  is  removed  by  chemical  decomposition  as  fast  as 
it  reaches  the  highest  point  of  the  capillary  system  of  cotton  fibres.  Any  other  cause 
which  would  effect  its  removal  in  as  complete  a  manner,  would  equally  produce  a  con- 
tinuous current.  The  same  explanation  applies  in  the  case  of  water  passing  through  a 
tissue  of  bladder  to  alcohol ;  for  as  soon  as  the  small  columns  which  percolate  through 
that  tissue  meet  with  the  alcohol,  they  are  removed  by  uniting  chemically  with  it,  and 
a  continuous  current,  therefore,  results.  The  current  of  alcohol  that  takes  place  in  the 
opposite  way  meets  with  a  similar  fate  ;  and  the  excess  of  one  of  these  currents  over 
the  other  determines  in  what  direction  the  hydrostatic  level  shall  change.  Even  the 
reputed  decompositions  brought  about  by  endosmosis  are  not  without  very  homely  and 
well-known  analogues.  The  greasy  wick,  when  dipped  into  a  lamp  containing  oil  and 
water,  removes  the  former  without  disturbing  the  latter. 

207.  It  has  been  shown  that,  to  exhibit  the  phenomena  of  endosmosis,  pores  of  a  cer- 
tain size  are  necessary ;  that  if  their  diameter  exceed  this,  the  mere  leakage  masks  ev- 
ery other  effect.  We  might  next  proceed  to  investigate  what  are  the  actual  dimensions 
demanded.  This  inquiry  is  not  alone  one  of  mere  curiosity,  but  meets  with  important 
applications  in  every  department  of  physiology;  and  the  problem,  if  successfully  solved, 
would  cast  a  great  deal  of  light  on  the  interstitial  communications  that  take  place  in 
every  part  of  organic  structure.  Vessels  of  an  excessive  degree  of  minuteness  creep 
through  the  finest  tissues,  which  might  almost  be  regarded  as  formed  by  the  interlacings 
of  these  narrow  capillary  tubes.  The  immediate  apertures  of  communication  between 
the  remote  fibrils  of  the  artery,  the  vein,  and  the  duct  of  any  gland,  are  of  an  indescri- 
bable smallness  ;  yet,  how  great  a  share  of  the  aggregate  of  the  actions  of  life  is  car- 
ried on  in  such  little  pores,  which  are  too  small  for  the  injection  of  the  anatomist  to 
reach,  or  even  for  microscopic  vision  to  descry. 

208.  These  pores  are,  however,  capable  of  approximative  admeasurement;  or,  at  least, 
their  dimensions  may  be  determined  within  limits  of  error.  The  method  by  which  this 
can  be  accomplished  essentially  depends  on  the  circumstance,  that  if  any  fluid  wiU  loet 
two  or  more  solids,  it  will  rise  in  capillary  pipes  formed  of  them  identically  to  the  same 
height,  no  matter  what  their  chemical  constitution  may  be,  provided  their  diameter  is 
the  same.  Thus,  water  will  rise  in  a  tube  of  glass,  of  serous  membrane,  or  in  a  straw, 
to  the  same  height,  if  the  diameters  be  alike. 

209.  It  has  been  stated  (199)  that  water  will  pass  into  a  chink  the  width  of  which 
is  not  more  than  the  half  of  a  millionth  part  of  an  inch,  under  the  condition  that  it  can 
wet  both  faces  of  the  chink.  Sir  I.  Newton  has  shown  {Optics,  b.  ii.,  p.  i.)  that  if  you 
lay  a  convex  lens  of  long  focus  on  a  glass  plane,  a  series  of  coloured  rings  surrounding  a 
central  black  spot  will  emerge  ;  and  it  is  known  from  simple  geometrical  principles,  that 
the  greatest  distance  between  the  two  glasses,  in  any  part  where  the  black  spot  appears, 
does  not  exceed  the  half  of  a  millionth  part  of  an  inch.    Yet,  if  a  drop  of  water  be 


I 


HYDRAULIC  CURRENTS.  (j5 

placed  between  the  glasses,  it  will  be  perceived  to  make  its  way  rapidly  to  the  central 
spot,  certain  optical  changes,  depending  on  its  superior  refractive  power  as  compared 
with  atmospheric  air,  accompanying  its  progress  ;  and  hence  we  infer,  that  if  a  chink 
or  cleft,  not  exceeding  the  half  of  a  millionth  of  an  inch,  occurred  in  an  animal  tissue, 
water  would  find  its  way  into  it. 

210.  In  vessels  of  large  diameter,  fluids  readily  adjust  themselves  hydrostatically,  and 
currents  set  in  any  direction  without  obstruction.  When  the  dimensions  of  the  con- 
taining vessels  become  very  small,  a  new  order  of  things  is  set  up,  and  the  particles 
have,  as  it  were,  to  obey  newly-created  forces.  In  large  masses,  the  action  of  gravity 
produces  the  leading  phenomena,  and  the  effects  of  all  the  molecular  forces  vanish. 
When  minute  quantities  are  operated  on,  the  action  of  gravity  diminishes,  and  fric- 
tion, cohesion,  capillary  action,  and  other  molecular  forces,  become  obvious.  The  me- 
chanical relations,  therefore,  of  small  and  large  quantities  are  totally  distinct :  hydro- 
static equilibrium,  which  is  effected  so  readily  in  larger  vessels,  is  accomplished  with 
more  difficulty  through  pores,  and  as  these  decrease  in  their  dimensions,  the  forces  of 
resistance  rapidly  increase.  Water,  at  all  pressures,  will  adjust  itself  hydrostatically 
with  great  readiness,  when  it  is  obstructed  by  a  porous  medium,  provided  the  pores  are 
of  sensible  size;  but  if  that  size  diminishes,  the  resisting  forces  continually  increasing, 
the  conditions  of  hydrostatic  equilibrium  are  fulfilled  with  more  difficulty,  and  at  last 
cease  to  be  fulfilled  at  all. 

211.  The  foregoing  remarks  enable  us  to  come  to  a  decision  in  reference  to  the  char- 
acter of  endosniosis,  as  indicated  in  (183).  We  perceive  that  this  force,  far  from 
being  the  attribute  of  organized  matter,  exhibits  its  phenomena  when  substances  whose 
inorganic  character  is  unquestionable  are  made  use  of.  A  variety  of  porous  minerals  may 
be  employed  in  lieu  of  organic  tissues  with  success;  and,  if  the  endosmosis  of  gases  be 
allowed  to  be  a  phenomenon  of  the  same  kind,  then  we  know  that  such  liquids  as 
water  may  be  employed  as  barriers  ;  a  peculiar  degree  of  porosity  is  required,  a  structure 
dense  enough  to  obstruct  readily  hydraulic  currents,  but  open  enough  to  allow  very 
small  columns  of  fluid  material  to  traverse  it.  A  crack  in  glass,  because  of  its  width, 
allows  too  great  a  freedom  of  motion ;  but  bladder,  peritoneum,  or  condensed  cellular 
tissue,  fulfil,  at  ordinary  pressures,  the  required  condition.  It  is  necessary,  too,  that 
the  liquids  under  trial  shall  wet  the  surface  of  the  solid ;  for  want  of  this  action,  water 
fails  to  pass  through  the  narrow  interstice  between  mercury  and  glass.  The  degree 
of  pressure  generated  either  during  the  action,  or  existing  at  the  commencement  of  the 
experiment,  is  an  important  element,  for  upon  it  depends  the  appearance  or  non-appear- 
ance of  the  phenomenon.  Thus,  at  ordinary  pressures,  bladder  will  exhibit  the  change 
of  level  when  water  passes  into  alcohol ;  but  if  pressure  on  one  side  of  the  membrane 
be  increased,  a  hydraulic  current  sets  through  it,  and  the  experiment  fails,  because  the 
success  of  the  result  depends  on  the  excess  of  the  molecular  over  the  hydraulic  current, 
and  in  this  case  the  latter  predominates.  The  relation  which  the  liquids  bear  to  each 
other  is  also  important ;  the  facility  with  which  they  unite  with  each  other,  and  therefore 
remove  each  other  on  transuding  through  the  barrier,  may  sometimes  make  up  for  an 
increased  size  of  the  pores. 

I 


66 


DEPOSITES  PRODUCED  BY  ENDOSMOTIC  CURRENTS. 


212.  There  is  no  absolute  diameter  at  which  a  pore  will  cease  to  permit  a  hydraulic  cur- 
rent to  pass  it,  and  the  phenomena  of  endosmosis  to  commence.  Size  is  but  one  of  the 
elements  involved  in  producing  this  action,  and  deviation  in  respect  of  it  may  be  often 
compensated  by  variations  in  the  other  conditions  ;  its  relation  to  pressure  is  not  unimpor- 
tant. Water  has  been  forced  through  the  interstices  of  gold,  and  melted  tin  through 
the  pores  of  solid  copper. 

213.  We  have  next  to  consider  the  different  cases  of  decomposition,  ostensibly 
brought  about  by  endosmosis. 

214.  If  litmus  water  be  placed  on  one  side  of  a  piece  of  bladder,  and  alcohol  on  the 
other,  tbe  water  will  forsake  the  colouring  matter  to  pass  through  the  bladder  and  unite 
with  the  alcohol!  This  experiment,  which  was  originally  cited  by  me  as  explanatory 
of  the  fact,  that  colouring  matter  in  the  intestines  could  not  give  its  peculiar  tint  to  the 
chyle,  does  away  with  one  of  the  most  important  objections  to  ihe  direct  absorption 
of  medicaments  by  the  lacteal  system.  In  estimating  its  true  value  among  the  facts 
now  under  consideration,  we  shall  find  that  it  is  very  far  from  supporting  the  hypothesis 
that  chemical  decompositions  can  be  brought  about  by  endosmosis.  There  is  no  proof 
that  the  colouring  matter,  though  permanently  suspended  in  the  water,  is  chemically 
united  with  it ;  analogies  would  lead  us  to  the  very  opposite  opinion.  All  that  can 
be  predicated  of  this  experiment  is,  that  it  exhibits  a  refined  kind  of  filtration,  which,  prob- 
ably, may  hereafter  become  of  considerable  importance  in  its  applications  in  the  arts  ;  as 
in  the  separation  of  colouring  matter  from  solutions,  or  the  preparation  of  medicines,  such 
as  the  vegetable  alkalies,  which  should  be  formed  from  colourless  solutions.  It  is  prob- 
able that  the  non-solubility  of  litmus  in  alcohol  is  not  without  its  influence  in  this  matter, 

215.  The  results  referred  to  in  (186),  (187),  (188),  may  all  be  classed  together.  They 
have  been  taken  as  proving  that  currents  may  set  in  determinate  directions  through  a 
membrane  ;  thus,  it  has  been  inferred  from  the  experiment  (187),  that  when  a  solution  of 
oxalic  acid  is  on  one  side  of  a  membrane,  and  lime-water  on  the  other,  the  acid  passes  free- 
ly through  the  pores,  but  a  passage  to  the  lime-water  is  denied.  It  has  been  thought  that 
an  action  of  this  kind  was  the  result  of  organization,  an  important  property  possessed 
by  membranes  only;  hence  it  has  been  inferred  that  tissues  j-Pwcd  of  the  transit  of 
bodies  in  certain  directions  through  tbem. 

216.  A  more  careful  investigation  of  the  circumstances  deprives  this  phenomenon 
of  all  its  mysterious  importance.  It  is  by  no  means  confined  to  tissues  or  organized 
matter  (197).  If  we  take  a  cupping-glass,  the  edge  of  which  is  truly  ground,  and, 
having  filled  it  with  lime-water,  place  it  upon  a  piece  of  clean  plate  glass,  then,  on 
pouring  a  solution  of  oxalic  acid  on  the  plate,  so  that  it  may  encircle  the  edge  of  the 
cupping-glass,  ii  will  be  perceived  that,  while  the  acid  solution  on  the  outside  remains 
clear  and  colourless,  innumerable  streams  of  oxalate  of  lime  will  pass  from  the  bottom 
of  the  glass,  and  rising  in  white  clouds,  render  the  solution  turbid.  Here,  surely,  we 
cannot  ascribe  any  organic  function  to  the  chink  between  the  two  pieces  of  glass;  yet 
the  current  apparently  sets  only  in  one  way,  and  exhibits,  to  all  intents  and  purposes,  a 
phenomenon  identically  the  same  with  that  referred  to  in  (187). 

217.  The  instances  here  referred  to  are  those  which  have  hitherto  attracted  attention. 


DEPOSITES  PRODUCED  BY  ENDOSMOTIC  CURRENTS.  (57 

Some  physicians  have  made  important,  but  unwarrantable  deductions  from  them.  Sacs 
formed  of  animal  tissues  have  been  supposed  to  be  competent  to  expel  saline  matters 
from  them  by  exosmose,  while  they  were  introducing  water  by  endosmose.  No  general 
rule  of  tbis  kind  will  apply,  nor  will  the  hypothesis  that  a  current  passes  in  one  direction 
only,  bear  the  test  of  a  close  examination.  It  has  been  stated  that,  when  oxalic  acid 
and  lime-water  are  separated  from  each  other  by  a  membrane,  a  precipitate  of  oxalate 
of  lime  takes  place  on  the  side  of  the  lime-water,  and,  therefore,  the  current  sets  from 
the  acid  to  the  lime-water,  but  none  in  the  opposite  direction.  The  same  occurs  in  the 
case  of  Prussian  blue,  which  is  always  found  on  one  side  of  the  membrane.  The  ac- 
tion is  correctly  reported,  but  the  inference  is  erroneous. 

218.  I  took  a  number  of  tubes,  open  at  both  ends,  and  tied  a  piece  of  bladder  on 
each.  They  served  to  contain  a  fluid  which  might  communicate  with  one  of  a  differ- 
ent kind,  capable  of  giving  a  precipitate,  contained  in  a  glass  receiver. 

a  Contained  sulphocyanate  of  potassa  in  the  tube,  and  solution  of  persulphate  ot 
iron  in  the  reservoir. 

h  Contained  solution  of  iodide  of  potassium  in  the  tube,  and  solution  of  bichloride  of 
mercury  in  the  reservoir. 

c  Contained  solution  of  oxalic  acid  in  the  tube,  and  lime-water  in  the  reservoir. 

d  Contained  solution  of  chloride  of  barium  in  the  tube,  and  dilute  sulphuric  acid  in 
the  reservoir. 

e  Contained  prussiate  of  potassa  in  the  tube,  and  persulphate  of  iron  in  the  reservoir. 
J  Contained  prussiate  of  potassa  in  the  tube,  and  protosulphate  of  iron  in  the  reservoir. 
g  Contained  bichromate  of  potassa  in  the  tube,  and  acetate  of  lead  in  the  reservoir. 
h  Contained  dilute  muriatic  acid  in  the  tube,  and  solution  of  nitrate  of  silver  in  the 
reservoir. 

i  Contained  solution  of  prussiate  of  potassa  in  the  tube,  and  sulphate  of  copper  in 
the  reservoir. 

219.  In  the  course  of  a  few  days  it  was  found  that,  in  the  arrangement  marked  «,  c, 
d,f,  the  level  within  the  tube  had  risen,  and  the  deposite  of  sulphocyanate  of  iron,  oxa- 
late of  lime,  sulphate  of  baryta,  and  Prussian  blue,  had  taken  place  within  the  tube ;  in 
the  arrangements  h  and  the  precipitates  of  biniodide  of  mercury  and  chromate  of 
lead  were  entirely  interstitial,  the  pores  and  cellular  tissue  of  the  membrane  being 
choked  with  them,  but  none  had  escaped  into  the  fluids  on  either  side  of  the  barrier ; 
the  membranes,  when  thus  injected,  formed  very  pretty  microscopical  objects;  the  level 
in  the  tube  h  had  risen,  but  that  in  g  had  fallen.  In  the  arrangements  e  and  h,  the  level 
in  the  tube  had  risen,  but  the  deposite  was  found  on  the  outside.  In  ^,  after  several 
days,  no  action  of  any  kind  could  be  perceived,  due,  perhaps,  to  the  unusual  thickness 
of  the  membrane. 

220.  From  the  aggregate  of  these  experiments  we  gather,  that  in  nearly  all  cases 
where  two  fluids,  which,  being  mixed,  give  rise  to  an  insoluble  precipitate,  are  separa- 
ted from  each  other  by  a  membrane,  the  precipitate  will  be  found  on  one  or  other 
of  the  sides  of  that  membrane,  but  hardly  ever  on  both  ;  sometimes  the  action  appears 
to  be  checked  by  the  choking  of  the  pores  and  interstices,  and  then  little  or  no  depos- 
ite is  found  on  either  side  of  the  tissue. 


68 


APPARENT  DECOMPOSITION  OF  METALLIC  SALTS  BY  MEMBRANES. 


221.  In  giving  an  explanation  of  these  curious  facts,  it  is  to  be  borne  in  mind,  that 
all  the  phenomena  treated  of  in  this  chapter  are  the  resuks  of  two  contemporaneous  cur- 
rents, endosuiosis  never  existing  without  exosmosc.  If  these  currents  are  established 
in  lluids,  which,  by  their  union,  give  rise  to  solid  matter,  its  deposite  may  occur  under 
all  the  forms  designated  (219),  (220).  If,  for  instance,  the  precipitate  be  a  light  mate- 
rial, and  one  of  the  currents  exceed  the  other  in  volume,  the  small  particles,  as  they  are 
formed,  are  drifted  by  the  current,  which  is  acting  under  the  greater  advantage,  and  the 
deposite  will  take  place  wholly  on  one  side.  This,  I  suppose,  is  the  mode  of  deposite 
of  oxalate  of  lime,  which  always  goes  with  the  greater  current.  The  chemical  change, 
or  union,  it  is  to  be  remembered,  takes  place  at  the  point  of  contact  of  the  two 
fluids,  which  is  necessarily  in  the  membrane  itself ;  there  they  neutralize  one  another, 
and  if  the  circumstances  of  the  experiment  permit,  the  more  pow^erful  current  carries 
before  it  the  precipitating  particles  as  fast  as  they  form,  and  the  excess  of  unneutralized 
material  in  it  produces  a  precipitate  of  the  same  kind  as  soon  as  it  mingles  with  the 
mass  of  fluid  on  the  side  of  the  membrane  towards  which  it  is  going.  But,  if  any  dis- 
turbing causes  intervene,  if  the  precipitated  matter  has  any  affinity  for  the  fibre  of  the 
membrane,  in  the  manner  of  a  dye,  or  if  it  be  too  bulky  to  pass  through  the  pores,  or 
too  ponderous  for  the  current  readily  to  move,  it  is  detained  on  the  spot  where  it  was 
generated,  and  in  a  very  short  time  the  tissue  becomes  choked ;  the  biniodide  of  mer- 
cury is  subject  to  these  circumstances.  A  number  of  disturbing  causes  will  often 
change  the  results  of  these  experiments:  when,  for  example,  the  precipitating  particles 
have  a  high  density,  their  weight  may  carry  them  in  a  direction  even  opposed  to 
the  stronger  current.  The  relative  specific  gravity  of  the  two  fluids  may  also  deter- 
mine the  course  in  which  the  particles  shall  go. 

222.  That  these  experiments  do  not  prove  that  membranes  have  a  predilection  for 
passage  in  certain  directions  through  them,  the  results  of  the  earlier  writers  are  suffi- 
cient to  show.  They  have  shown  that  when  Prussian  blue  is  deposited  in  this  way, 
sometimes  the  precipitate  is  towards  the  salt  of  iron,  and  sometimes  towards  the  prus- 
siate  of  potash,  the  direction  it  takes  being  often  influenced  by  very  slight  causes.  Nay, 
even  analogous  actions  may  be  exhibited  without  using  any  membrane,  barrier,  or  ob- 
struction; if  into  a  half-ounce  vial  a  quantity  of  strong  sulphuric  acid  is  poured,  and 
upon  that  a  solution  of  chloride  of  calcium,  so  that  the  two  fluids  may  intermix  as  little 
as  possible,  in  the  course  of  a  few  days  it  will  be  seen  that,  as  the  fluids  slowly  difliise 
into  each  other,  the  sulphate  of  lime  is  deposited  entirely  in  the  supernatant  solution, 
and  none  in  the  strong  acid,  a  result  unquestionably  depending  upon  their  relative  spe- 
cific gravity  and  cohesion. 

223.  Sections  (188),  (189),  contain  examples  of  what  has  been  termed  in  these  papers 
decompositions  of  a  certain  sort.  In  relation  to  the  first  of  these,  it  is  by  no  means 
clearly  proved  that  oxygen  really  does  leave  the  nitrogen  in  the  atmosphere  to  go 
through  the  barrier.  The  two  gases  may  respectively  pass  into  each  other,  as  atmo- 
spheric air  and  nitrogen.  It  is  true  that  the  more  probable  mode  of  passage  is  that 
assumed  in  the  section  quoted.  If  so,  it  does  not  even  follow  that  a  real  chemical  de- 
composition happens,  for  there  is  much  reason  to  doubt  whether  atmospheric  air  is 


I 


APPARENT  DECOMPOSITION  OF  METALLIC  SALTS  BY  MEMBRANES.  (59 

Itself  a  compound.  The  same  observation  applies,  to  a  Certain  extent,  to  the  result 
(189),  which  is  an  effect  analogous  to  that  produced  by  a  few  fibres  of  greasy  cotton 
when  dipped  into  a  mixture  of  oil  and  water,  as  in  a  common  lamp :  effects  which  are 
totally  distinct  from  chemical  decomposition. 

•224.  The  experiment  referred  to  in  (190)  is  apparently  the  most  important  of  these 
cases  of  tissue  action.  "  A  tube  of  suitable  dimensions,  having  one  of  its  extremities 
closed  with  a  piece  of  bladder,  a  h,  Jig.  25,  and  filled  to  a  certain  height,  e  e,  with  pure 
water,  into  which  a  few  iron  nails  were  dropped,  was  immersed  in  a  solution  of  sulphate 
of  copper.  In  the  course  of  a  few  days  a  deposite  of  metallic  copper  was  found  on  the 
surface  of  the  membrane  towards  c,  but  none  on  the  inner  side,  or  upon  the  iron  nails. 
A  number  of  other  metallic  salts,  such  as  acetate  of  lead,  nitrate  of  silver,  &c.,  afford 
similar  results." 

225.  Certainly  this  is  a  remarkable  phenomenon,  if  the  conditions  under  which  it 
occurs  are  accurately  detailed.  The  operation  and  laws  of  chemical  affinity  would 
lead  us  to  ascribe  the  decomposition  of  sulphate  of  copper  to  the  action  of  the  iron ; 
but  then  it  would  appear  that  those  same  laws  require  the  metallic  precipitate  to  be 
deposited  on  the  disturbing  metal ;  here,  however,  it  is  found  that  the  deposite  really 
occurs  on  the  membranous  tissue. 

226.  On  placing  on  one  side  of  a  membrane  pure  water,_and  on  the  other  sulphate 
of  copper,  or  any  other  salt  reputed  to  be  capable  of  decomposition  under  the  circum- 
stances given,  no  chemical  action  whatever  occurred,  in  many  days,  at  all  analogous  to 
the  phenomena ;  but  the  water  passed  out  of  the  tube  into  the  copper  solution  with 
considerable  rapidity. 

227.  On  repeating  the  experiment,  as  given  in  (224),  in  the  course  of  six  days  the 
results  were  as  follows  :  A  great  disturbance  of  hydrostatic  level  had  occurred,  there 
being  an  accumulation  in  the  tube  containing  the  iron  nails  ;  the  under  side  of  the  blad- 
der was  coated  to  a  considerable  thickness  with  bright  metallic  copper,  and  a  small 
portion,  in  a  pulverulent  form,  was  found  on  the  inner  edge,  and  here  and  there  patches 
were  discovered  coating  the  surface  of  the  iron  nails,  and  minute  veins  of  copper  pass- 
ing through  the  bladder.  In  many  repetitions  of  this,  the  same  results  were  uniformly 
observed. 

228.  These  numerous  experiments  showed  that  in  all  cases  there  was  a  perfect 
metallic  communication  from  the  iron  nails,  by  means  of  veins  of  copper,  through 
the  bladder,  with  the  cupreous  mass  on  the  under  side.  A  piece  of  zinc,  z,  was  there- 
fore suspended  in  the  tube,  a  a,  fig.  26,  in  pure  water,  at  a  distance  of  three  quarters 
of  an  inch  above  the  bladder,  and  the  arrangement  exposed  to  a  solution  of  acetate  of 
lead.  For  several  hours  no  chemical  action  whatever  occurred,  the  water  exosmosing 
towards  the  metallic  solution,  and  the  level  in  the  tube,  of  course,  falling.  At  the 
same  time,  a  small  portion  of  solution  of  acetate  of  lead  passed  into  the  pure  water, 
as  was  shown,  after  the  lapse  of  twenty  hours,  by  the  formation  of  some  thin  filaments 
of  lead  on  the  lower  edge  of  the  zinc  ;  these  kept  increasing  in  length,  and  finally  reach- 
ed the  bladder  ;  soon  after,  they  appeared  to  have  made  their  way  through  the  pores 
of  that  structure,  and  then  the  usual  deposite  occurred  on  the  under  side  of  the  mem- 


70 


APPARENT  DECOMPOSITION  OF  METALLIC  SALTS  BY  MEMBRANES. 


brane :  simultaneously,  the  hydraulic  current  changed,  and  the  level  of  the  fluid  in  the 
tube  began  to  rise. 

229.  A  semicircular  piece  of  very  thin  sheet-iron  was  placed  on  the  inside  of  the 
bladder;  it  caused,  in  a  very  short  time,  a  copious  precipitate  of  the  copper  on  that 
part  of  the  bladder  on  which  it  lay,  the  remainder  being  free  from  metallic  deposite.  A 
very  well-marked  change  of  hydrostatic  level  occurred.  On  leaning  the  tube,  the  thin 
piece  of  sheet-iron  was  found  to  adhere  to  the  bladder ;  and,  on  tearing  away  the 
cupreous  deposite,  the  little  veins,  communicating  through  the  porous  texture,  were  vis- 
ible without  a  lens. 

230.  Lastly,  when  the  relative  position  of  the  arrangement  was  reversed,  the  pieces 
of  iron  being  placed  below  the  membrane,  and  the  copper  solution  above,  so  that  the 
little  filaments  of  metallic  matter  could  not  reach  the  tissue  on  account  of  their  weight, 
but  fell  down  ;  or  when  they  were  destroyed  by  mechanical  means  as  fast  as  they 
formed,  no  deposite  ever  occurred  on  the  bladder. 

231.  The  experiments  (228)  and  (230)  especially,  and  the  other  experiments  incident- 
ally, prove  that  a  perfect  metallic  communication  must  extend  from  the  decomposing 
metal  to  the  under  side  of  the  membrane,  through  the  very  substance  of  that  structure  ; 
and  this  observation  enables  us  to  give  the  true  theory  of  the  process.    Let  a  a  a  a  {Jig. 
27)  be  a  tube,  divided  in  some  portion  of  its  length  by  a  membranous  partition,  h  h,  on  one 
side  of  which.  A,  pure  water  is  placed,  containing  the  decomposing  metal,  c,  and  on  the 
other  side  B,  the  metallic  solution  intended  to  be  decomposed.    Owing  to  the  obstruc- 
tion caused  by  the  membranous  diaphragm,  the  diffusion  of  the  water  into  the  solution, 
and  of  the  solution  into  the  water,  will  be  retarded ;  in  the  course  of  time,  however,  it 
will  take  place.    Then,  as  soon  as  any  of  the  metallic  salt  has  reached  the  lower  part 
of  the  decomposing  metal,  reduction  ensues  in  virtue  of  the  common  play  of  chemical 
affinity,  the  first  portion  of  the  metal  thus  eliminated  adhering  to  the  surface  of  the  de- 
composing metal,  and  forming  with  it  an  active  voltaic  couple  ;  the  decomposing  metal 
being  thereby  rendered  positive,  and  the  eliminated  metal  negative.    Of  course,  the  re- 
duction of  the  dissolved  metal  continues,  the  aggregate  of  filaments  extending  itself  down 
by  its  weight  towards  the  diaphragm;  and  as  the  solution  becomes  stronger  and  stronger 
the  nearer  the  filaments  approach  the  bladder,  so  the  metallic  deposite  becomes  more 
abundant.    No  obstruction  is  experienced  in  passing  through  the  pores  of  the  diaphragm, 
because  the  metallic  solution  is  still  present  there,  and  is  reduced.    But  the  moment  any 
one  of  the  metallic  veins  has  penetrated  to  the  under  side  of  the  membrane,  the  process 
goes  on  with  tenfold  activity.    The  metallic  deposite,  when  it  first  started  from  the  de- 
composing metal,  was,  perhaps,  merely  a  capillary  thread,  for  the  solution  through  w  hich 
it  was  passing  was  so  weak  that  a  greater  quantity  could  not  be  presented  ;  as  it 
neared  the  upper  surface  of  the  diaphragm,  it  became  of  stouter  substance,  because  the 
solution,  being  stronger,  the  supply  was  more  abundant;  and  now,  having  passed  the 
barrier  and  reached  the  strong  metallic  solution,  the  deposite  is  at  once  at  a  maximum. 
Hence  we  account  for  the  circumstance  that  the  reduction  of  the  metal  chiefly  takes 
place  on  the  under  side  of  the  membrane. 

232.  It  may  here  be  asked.  How  is  the  acid  in  the  metallic  solution  conveyed  from  a 


TRUE  THEORY  OF  IT.  7] 

distance  to  the  decomposing  metal,  so  as  to  unite  with  it  ?  It  was  to  encounter  this 
query,  and  to  answer  it,  that  I  have  been  induced  to  examine  so  much  in  detail  the 
whole  of  this  process.  There  is  no  physical  dififcrence  between  the  deposites  here 
spoken  of,  and  those  known  formerly  to  chemists  under  the  name  of  Arbor  Saturni  and 
Arbor  Diansc ;  for  the  bladder  or  other  membrane  being  freely  penetrated,  impresses 
no  sort  of  action  or  change  on  the  process  in  any  wise.  The  same  difficulty  here  met 
with  occurs  in  all  such  cases  of  metallic  precipitations.  For  example,  in  making  the 
lead  tree,  by  the  action  of  a  piece  of  zinc  on  some  saturnine  solution,  the  particles 
of  lead  are  often  evolved  at  a  distance  of  six  or  eight  inches  from  the  decomposing 
metal.  How,  it  may  be  asked,  is  the  acid  transported  in  this  case?  More  than  a  cen- 
tury ago,  La  Condamine  endeavoured  to  account  for  this,  but  the  state  of  chemical 
knowledge  at  that  time  did  not  enable  him  to  assign  the  true  reason,  and  he  was  forced 
to  infer  that  currents  actually  traversed  the  solution,  making  their  way  to  the  decom- 
posing metal.  All  decompositions  brought  about  by  the  agency  of  the  voltaic  battery 
are  cases  in  point.  The  elements  of  water  are  separately  evolved  when  the  electrodes 
are  many  inches  apart,  and  the  decomposition  of  the  very  salts  here  under  considera- 
tion occurs  in  the  same  way.  It  is  now  generally  admitted  that  these  decompositions 
do  not  occur  to  solitary  particles  at  a  time  ;  but  a  chain  of  them,  extending  from  one 
electrode  to  the  other,  undergoes  simultaneous  decompositions  and  recompositions,  a 
process  which  eventuates  in  the  elimination  of  the  separate  ingredients  at  opposite  ex- 
tremities of  the  chain.    This  explanation  applies  to  the  case  here  considered. 

233.  In  support  of  the  explanation  here  given,  the  phenomenon  of  change  of  hydro- 
static level  affords  a  very  powerful  evidence.  It  was  stated  (228)  that,  previous  to  the 
establishment  of  a  voltaic  current  from  one  side  of  the  bladder  to  the  other,  the  hydrau- 
lic current  set  from  the  water  towards  the  solution,  and,  therefore,  the  level  in  the  tube 
kept  falling ;  but  so  soon  as  the  voltaic  current  passed  through  the  barrier,  the  hydrau- 
lic current  changed,  and  the  level  in  the  tube  rose  rapidly.  Similar  changes  of  level 
have  been  noticed,  when  voltaic  currents  pass  through  membranes,  by  Mr.  Porrett  and 
other  chemists  ;  they  appear  to  be  certain  indexes  of  that  molecular  interchange  (not 
current)  which  eventuates  in  polar  decomposition. 

234.  I  therefore  reject  entirely  this  experiment,  as  having  anything  whatever  to  do 
with  the  phenomena  of  tissue  action,  or  as  affording  an  example  of  the  powerful  de- 
composing agency  of  membranes.  It  has  been  considered  in  detail,  inasmuch  as  it  is 
as  necessary,  in  studying  the  elements  of  a  science  which,  like  physiology,  is  yet  in  its 
infancy,  to  discard  whatever  is  irrelevant  or  erroneous,  as  to  point  out  what  is  pertinent 
and  true. 

235.  After  a  very  full  examination  of  the  question  before  us,  it  is  not  to  be  concealed 
that  the  decision  to  which  we  are  constrained  to  come  is  entirely  unfavourable  to  the 
opinion  commonly  held  as  respects  endosmosis.  To  rank  it  as  a  peculiar  power  is  un- 
questionably erroneous ;  to  regard  it  as  a  manifestation  of  organization,  or  the  attribute 
of  organized  structures,  is  equally  so.  Ever  since  the  leading  phenomena  were  made 
known  by  Porrett,  Fischer,  Gustave  Magnus,  and  Dutrochet,  they  have  been  extensively 
applied  in  the  elucidation  of  kindred  actions  in  the  vital  frame ;  the  causes  of  a  variety 


72 


ENDOSMOSIS  IS  NOTHING  MORE  THAN  COMMON  CAPILLARY  ATTRACTION. 


of  diseases  have  been  explained,  as  also  the  rationale  of  some  modes  of  treatment.  A 
premature  appUcatiou  of  principles  ill  understood  is  to  be  deprecated.  On  the  discov- 
ery of  a  new  fact,  it  is  not  always  easy  to  determine  its  relation  and  position  in  the 
chain  of  knowledge  ;  a  confused  idea  is  generally  entertained  of  it ;  nor  is  it  until  time 
and  experience  have  abstracted  from  it  whatever  was  mysterious  and  doubtful,  that  we 
see  clearly  its  true  locality  in  respect  of  other  facts,  how  they  bear  upon  it,  and  how  it 
bears  upon  them. 

236.  We  conclude,  therefore,  that  endosmosis  is  not  a  new  power,  nor  does  it  bear 
any  peculiar  relation  to  organization  ;  but  that  it  is  a  manifestation  of  capillary  attraction. 

237.  That,  so  far  as  the  examination  in  this  memoir  has  extended,  tbere  is  no  case 
upon  record  in  which  endosmosis  has  effected  a  real  and  undoubted  chemical  decom- 
position ;  that  several  cases  of  such  reputed  change  depend  plainly  on  the  action  of 
other  agents  ;  and  hence,  that  those  reported  instances  of  the  production  of  secreted 
fluids  by  dead  membranes,  through  this  power,  are  fanciful  illusions. 

238.  The  experiments  here  considered  are  such  as  were  conceived  to  be  the  most 
important,  and  those  generally  referred  to,  as  substantiating  conclusions  which  conflict 
with  that  here  given.  If  others  be  known  not  subject  to  these  or  similar  strictures,  I 
am  ignorant  of  them  ;  such  may,  by  chance,  hereafter  be  found  ;  the  decision  now  given 
refers  to  what  have  been  regarded  as  actual  proofs,  and  not  to  contingent  evidence. 
That  chemical  changes  of  all  kinds  occur  in  tissues  and  glands,  is  not  to  be  doubted  ;  but 
we  must  not  confound  together  a  change  effected  in  a  tissue,  and  one  effected  through 
it.  Urine  is  readily  separated  from  arterial  blood  in  the  kidney ;  yet  would  any  one 
expect,  on  placing  blood  njwn  a  kidney,  that  urine  would  drop  through  it  ?  A  candid 
examination  of  many  of  the  fashionable  applications  of  endosmosis  to  physiological  func- 
tions will  discover  no  wide  difference  between  them  and  this  hypothetical  case. 

239.  It  is  well  known  that  those  who  first  cultivated  this  department  of  science, 
viewed  it  as  a  case  of  electrical  action.  In  this  they  did  not  go  far  astray  ;  the  ma- 
chinery being  erroneous,  though  the  principle  was  true.  Capillarity  is  unquestionably 
an  electro-statical  phenomenon,  and  hence  will  hereafter  come  to  be  intimately  allied 
with  chemistry.  An  important  and  extensive  series  of  effects,  which  now  pass  as  in- 
stances of  electrical  attraction,  will  be  assigned  to  it ;  such  as  the  adhesion  of  colouring 
matter  and  dyes  to  cloth,  the  silvering  to  a  looking-glass,  the  solution  of  salts,  and  gen- 
erally all  cases  of  union  where  the  uniting  bodies  lose  none  of  their  prominent  charac- 
teristics. 


OBJECT  OF  THE  MEMOIR.  73 


CHAPTER  VIII. 

ON  THE   USE   OF  A  SECONDARY  WIRE  AS  A  MEASURE   OF   THE   RELATIVE   TENSION  OF 

ELECTRIC  CURRENTS. 

{From  the  London  and  Edinburgh  Philosophical  Magazine  for  October  and  November,  1839.) 

Contents  :  Object  of  the  Meynoir. — Action  of  a  Secondary  Wire. — Vescriiition  of  the 
Torsion  Galvanometer. — Resistance  of  the  Secondary  Wire  under  Variations  of  Ten- 
sion.— Condition  of  the  Current  never  changes. —  Tension  rises  with  Length  of  Wire 
and  with  Distance  of  Plates. — Kelation  between  Quantity  and  Tension. —  Theory  of 
Tension  of  the  Voltaic  Battery. — Knowro  Methods  of  increasing  Tension  of  Currents. 
— General  Law. — Case  of  Thermo- Elect?-icity  and  Machijie  Tllectricity. —  Voltaic  Spark 
before  Contact  in  Vacuo. 

240.  It  is  the  object  of  this  memoir  to  estabUsh  the  following  propositions  : 

1st.  That  by  means  of  a  secondary  wire,  we  may  always  determine  the  relative  ten- 
sion of  electric  currents. 

2d.  That  there  is  reason  to  doubt  whether  the  processes  usually  supposed  to  affect 
the  condition  of  an  electric  current  are  ever  attended  with  any  such  result ;  but  that, 
when  changes  have  apparently  taken  place,  it  is  probable  that  they  may  be  directly 
traced  either  to  a  disturbance  at  the  place  of  generation,  or  to  the  development  of  other 
currents  of  a  different  character,  the  primary  current  itself  remaining  unchanged. 

3d.  That  there  are  two  different  methods  of  accomplishing  these  disturbances,  and 
thereby  of  raising  the  elastic  force  of  a  current :  1st.  That  tension  may  be  augmented 
by  the  sacrifice  of  quantity;  Volta's  plan  of  a  reduplicated  series,  and  Henry's  riband 
coil,  in  its  condition  of  equilibrium,  being  examples :  2d.  By  the  introduction  of  new 
affinities  in  the  exciting  cells  ;  batteries  charged  with  nitrosulphuric  acid  or  sulphate  of 
copper  are  examples. 

4th.  That  the  law  which  regulates  the  connexion  of  this  diminution  of  quantity  or 
condensation  with  the  increase  of  tension  is  the  same  as  that  which  regulates  the  anal- 
ogous phenomena  of  ponderable  elastic  fluids. 

241.  Incidentally,  the  examination  of  certain  other  points  will  be  entered  upon:  for 
example,  a  brief  consideration  of  Lenz's  law  of  the  conducting  power  of  wires  ;  this,  it 
will  be  shown,  holds  not  only  in  the  case  of  Faradian  currents,  but  in  the  direct  cur- 
rents from  hydro-electric  and  thermo-electric  pairs,  as  has  been  advanced  by  some 
philosophers,  but  denied  by  others. 

242.  The  terms  tension,  intensity,  tensile  effect,  &c.,  have  had  very  different  signifi- 
cations attached  to  them.  From  this  circumstance  a  great  deal  of  confusion  has  arisen, 
and  it  is  one  of  the  causes  of  that  diversity  of  opinion  and  contrariety  of  theory  which 
obtain  in  the  elementary  parts  of  the  science  of  electricity.  For  example,  Dr.  Faraday 
appears  to  use  the  words  tension  and  intensity  as  synonymous,  expressive,  as  it  were,  of 

K 


74 


ACTION  OF  A  SECONDARY  WIRE. 


elastic  force,  chemical  authors  generally  adopting  the  same  signification:  "The  re- 
moteness from  the  unexcited  state,  a  condition  expressed  by  the  terms  tension  or  inten- 
sity." "  By  tension  or  intensity  is  meant  the  energy  or  effort  with  which  the  current 
is  impelled." — (^Turner,  Elem.  Cliem.') 

243.  This  confusion  of  terms  leads  to  a  confusion  of  facts  of  a  nmch  more  serious 
kind.  English  electricians  uniformly  state  that  the  magnetic  needle,  deviating  in  the 
neighbourhood  of  a  current,  takes  no  note  whatever  of  the  intensity  of  that  current. 
Continental  writers,  almost  without  exception,  regard  the  deviation  as  a  function  of  the 
intensity,  and  the  statements,  therefore,  appear  discordant.  While  the  effect  is  thus  dif- 
ferently described,  all  agree  as  to  the  facts  of  the  case.  In  what  follows,  the  term  tension 
will  be  used  as  expressive  of  the  elastic  force  of  the  current,  that  power  by  which  it  is 
enabled  to  pass  a  resisting  medium ;  the  term  intensity  will  be  strictly  confined  to  the 
acceptation  in  which  writers  on  analytical  mechanics  use  it.  "By  the  intensity  of  a  force, 
we  understand  its  greater  or  lesser  capacity  to  produce  motion"  {Doucharlaty,  and  in 
the  case  before  us,  the  intensity  will  be  regarded  as  a  function  of  the  quantity  and  ten- 
sion conjointly.  Thus,  the  deviation  of  a  magnetic  needle  does  not  indicate  the  ten- 
sion, but  the  intensity  of  a  current. 

244.  Suppose,  now,  we  had  a  current  of  electricity  passing  under  a  certain  tension, 
along  a  channel  of  conduction,  as  a  bar  of  large  dimensions,  and  were  suddenly  to  in- 
terpose in  some  part  of  its  path  a  resisting  obstacle,  as,  for  example,  a  slender  wire  ;  it  is 
obvious  that  a  certain  portion  of  the  current  would  pass  the  barrier,  a  portion  deter- 
mined partly  by  the  character  and  dimensions  of  the  wire,  and  partly  by  the  tension  or 
elastic  force  of  the  current.  Let  the  wire  under  all  circumstances  be  the  same,  the  ab- 
solute quantity  of  electricity  be  constant,  but  the  tension  thereof  vary.  Now,  as  the 
tension  increases,  the  quantity  that  passes  the  resisting  wire  will  also  increase,  and  as  the 
one  diminishes,  so  will  the  other  too.  Under  these  circumstances,  the  absolute  quantity 
that  passes  will  always  be  an  increasing  function  of  the  tension;  and  as  this  quantity  is 
under  all  circumstances  measureable  by  the  deviations  of  the  magnetic  needle,  or  by  the 
voltameter,  these  instruments  may  be  used  to  determine  the  tension,  by  making  quan- 
tity indirectly  the  measure  thereof 

245.  If,  therefore,  we  send  a  certain  quantity  of  electricity,  as  100  parts,  to  a  resist- 
ing wire,  and  find  that  of  these  50  parts  can  pass  the  obstruction,  we  may  assume  such 
a  current  to  have  a  higher  tension  than  one  containing  the  same  absolute  quantity,  of 
which  only  30  could  pass ;  and  to  have  a  much  lower  tension  than  one,  of  which  70, 
80,  or  90  parts  could  pass.  In  all  these  cases,  the  amount  per  cent,  of  the  main  cur- 
rent which  passes  the  resisting  wire  may  be  taken  as  the  representative  of  the  tension 
of  that  current. 

24G.  This  obstructing,  resisting  wire,  T  call  a  secondary  wire. 

247.  But  it  is  plain  that  this  amount  per  cent.,  of  which  I  am  speaking,  in  introdu- 
cing this  fundamental  proposition,  is  nothing  more  than  the  ratio  which  exists  between 
the  quantities  passing  the  large  and  the  little  wires  respectively.  By  dividing,  there- 
fore, the  quantity  that  passes  the  secondary  wire  by  the  quantity  that  passes  the  large 
wire,  we  shall  have  a  numercial  representative  of  the  relative  tension  of  the  current  un- 
der consideration. 


DESCRIPTION  OF  THE  TORSION  GALVANOMETER.  75 

248.  Let  us  take  an  example  :  a  single  pair  of  plates  developed  a  current  of  electri- 
city, which,  when  measured  at  the  torsion  balance,  was  found  equal  to  20  degrees ;  on 
subjecting  this  current  to  a  secondary  wire,  7  degrees  passed  it.  Its  tension  might, 
therefore,  be  represented  by  -3500.  A  second  pair  was  now  added  in  conformity  to 
the  first,  31  parts  passing ;  but  when  subjected  to  the  secondary  wire,  18  were  indicated. 
The  tension  had  now  become  -5806:  in  the  same  way,  by  adding  three  more  pairs,  the 
tension  rose  to  '6346. 

249.  It  must  now  be  borne  in  mind,  that  the  numerical  determinations  thus  procured 
are  entirely  conventional ;  their  absolute  value  depends  upon  the  resistance  of  the 
secondary  wire,  and  they  therefore  only  express  the  relative  condition  of  different 
currents. 

250.  As  a  considerable  advantage  will  be  gained,  and  much  repetition  avoided,  by 
here  indicating  the  mode  adopted  for  procuring  the  following  measures,  I  shall  describe 
at  once  some  modifications  and  additions  which  are  necessary  in  the  torsion  balance, 
the  instrument  generally  employed. 

251.  The  voltameter  has  of  late  come  much  into  use  in  investigations  of  this  sort, 
but  when  compared  with  the  torsion  balance,  the  latter  is  mucli  more  speedy  and  cer- 
tain in  its  indications,  and  should  generally  be  preferred.  In  point  of  fact,  the  indica- 
tions of  the  two  instruments  are  entirely  of  a  diffi^rent  character  ;  the  magnetic  needle 
shows  the  quantity  of  electricity  that  is  passing  in  each  indivisible  portion  of  time,  the 
voltameter  the  quantity  that  has  passed  at  the  end  of  a  finite  time.  In  the  conditions 
of  the  action  of  the  one,  time  enters  as  an  element,  in  the  other  it  does  not. 

252.  By  applying  a  glass  thread  to  the  needle,  the  late  Dr.  Ritchie  greatly  improved 
the  accuracy  and  general  utility  of  the  galvanometer  ;  but  even  with  that  addition, 
unless  certain  precautions  are  taken,  the  instrument  will  not  work  satisfactorily  ;  the 
motions  of  the  needle  are  too  versatile,  and  the  tremulous  state  of  vibration  into  which 
it  may  be  thrown  are  insuperable  barriers  to  accuracy  of  measurement.  A  cylindrical 
trough  filled  with  water  is  a  perfect  and  admirable  remedy  for  these  difficulties. 

253.  Another  difficulty,  which  is  very  generally  overlooked,  is  the  excentric  position 
into  which  the  thread  is  liable  to  be  cast,  when  the  upper  micrometer  has  moved.  The 
construction  of  the  instrument  requires  that  the  axis  of  motion  of  the  upper  micrometer, 
the  axis  of  the  glass  thread,  the  axis  of  the  spindle  carrying  the  needles,  and  the  vane, 
should  be  in  the  same  vertical  straight  line,  through  whatever  arc  the  micrometer  may 
have  moved.  Now  it  would  be  very  difficult  to  accomplish  this  by  any  system  of 
adjustments. 

254.  Whether  the  instrument  is  arranged  with  one  or  several  needles,  or  whether  it 
has  a  coil  or  merely  a  single  strap,  the  vertical  distance  from  the  coil  or  strap,  when 
the  index  is  brought  to  zero,  ought  under  no  circumstances  to  vary. 

255.  In  a  climate  as  hot  as  that  in  which  the  following  experiments  were  made,  one 
of  the  most  unpleasant  deviations  depends  on  the  thread  wrenching  in  the  wax,  which 
is  used  to  fasten  it  to  the  needles  at  one  end,  and  to  the  micrometer  at  the  other;  when 
the  wax  softens,  and  the  thread  is  moved  through  several  degrees,  it  is  not  the  free  part 
alone  that  undergoes  torsion,  but  also  that  which  is  in  the  wax;  hence  arises  an  error  as 


76 


DESCRIPTION  OF  THP]  TORSION  GALVANOMETER. 


respects  the  zero  point.  This  I  have  always  avoided,  by  ascertaining  the  zero  at  the 
beginning  and  close  of  each  experiment. 

256.  After  having  had  some  experience  with  voltameters,  deflecting  galvanometers, 
&c.,  1  am  induced  to  describe  the  instrument  used  in  these  experiments,  for  it  will  ena- 
ble those  who  are  not  accustomed  to  the  torsion  balance  to  execute  measures  very 
easily,  which  they  might  otherwise  ineffectually  attempt. 

257.  A  A,  B  B  {Jig.  28),  is  a  glass  jar,  16  inches  high,  open  at  both  ends;  at  A  A 
it  is  2?  inches  in  diameter,  at  B  B  6  inches  ;  it  rests  upon  a  piece  of  wood  8  inches  by 
10.  A  strap  of  stout  sheet-copper,  e  f  f  e,  1  inch  wide  and  15  long,  is  bent  into  the 
form  indicated  ;  its  extremities  at  e  e  being  let  into  the  wood,  and  bearing  mercury 
boxes,  D  D.  The  central  part  of  this  strap,  from  /  toy^  is  placed  horizontally,  and  has 
a  circular  aperture  and  side  gap,  as  is  shown  in^o-.  29,  a  a,  through  which  the  spindle 
carrying  the  needle  can  be  passed,  and  works. 

258.  The  upper  extremity  of  the  jar,  A  A,  is  accommodated  with  a  divided  circle,  in 
the  centre  of  which  the  key  G  works :  this  key  is  ground,  like  a  stopcock,  to  a  slightly 
conical  figure ;  it  therefore  revolves  very  truly  without  any  shake :  it  is  drilled  longitu- 
dinally to  admit  the  passage  of  the  glass  thread,  which  is  secured  in  it  by  means  of  a 
perforated  straw  and  a  drop  of  sealing-wax. 

2  'j9.  The  other  extremity  of  the  thread  enters  a  little  tubular  perforation  in  the  ivory 
axis,  n  n',  and  is  also  secured  therein  by  wax.  Only  one  needle  is  used ;  it  is  lozenge- 
shaped,  and  is  4^  inches  long.  Besides  carrying  this  needle,  the  ivory  axis  extends  an 
inch  and  a  half  below  it,  and  in  a  slit  at  its  lower  extremity,  confines  a  parallelogram 
of  stout  tinfoil,  r  r,  an  inch  wide,  and  2f  long.  When  in  use,  this  vane  of  tinfoil  works 
in  a  glass  cup,  k  k,  3^  inches  in  diameter,  which  is  filled  with  water. 

260.  One  of  the  chief  improvements  in  the  instrument  is  connected  with  the  needle, 
and  the  axis  on  which  it  works.  The  latter  is  a  small  cylinder  of  ivory  ;  it  has  two  flat 
faces  filed  upon  it,  corresponding  to  the  direction  of  the  needle.  On  each  of  these 
faces,  as  is  represented  in  {^g.  30),  is  drawn  a  vertical  line,  and  a  little  to  the  right  of 
it  are  placed  five  dots.  The  polar  extremities  of  the  needle  are  accommodated  with 
two  upright  wires,  J}  p' ,  p  2^ ,  an  inch  long,  which  serve  as  indexes;  and  at  a  distance 
of  10  or  15  inches,  in  the  magnetic  meridian,  a  plate  of  metal,  not  shown  in  the  figure, 
with  a  small  hole  in  its  centre,  is  placed,  to  be  used  as  a  sight.  When  an  observation 
is  to  be  made,  the  experimenter  adjusts  this  sight  in  front  of  the  instrument,  either  on 
its  north  or  south  side ;  and  on  looking  through  it,  as  soon  as  the  needle  moves,  he 
sees  the  index,  7-'',  traverse  before  the  scale  on  the  axis.  There  is  no  shake  or  vil)ra- 
tion,  even  though  any  one  should  cross  the  floor  or  jar  the  table,  for  the  index  and 
the  scale  equally  participating  in  all  these  disturbances,  the  motion  i^  almost  as  steady  as 
that  of  a  shadow  on  a  sundial ;  the  vane  of  tinfoil  does  not  in  the  least  interfere  with 
the  accuracy  of  indication,  but  effectually  stops  the  oscillations,  and  the  utmost  accuracy 
may  be  obtained,  by  previously  giving  the  index, 7?',  a  slight  bend  out  of  the  vertical 
line,  and  using  the  five  dots  as  a  diagonal  vernier. 

261.  In  the  following  memoir,  it  will  be  seen  that  the  terms  primary  and  secondary 
wire  are  occasionally  used,  the  former  in  a  somewhat  extended  sense :  I  mean  by  it 


RESISTANCE  OF  THE  SECONDARY  WIRE  UNDER  VARIATIONS  OF  TENSION. 


77 


not  only  the  thick  polar  wires  that  come  from  the  electromoter,  those  which  were  used 
being  one  fifth  of  an  inch  thick,  but  include  the  electromoter  itself,  no  matter  what 
its  character  may  be — if  a  hydro-arrangement,  the  plates,  exciting  liquid,  &c.  The 
secondary  wires  are  simply  long  or  slender  wires,  to  obstruct  the  current;  of  these  I 
have  occasionally  used  two,  the  first  47  inches  long,  the  second  290  :  they  are  of 
copper,  one  foot  of  which  weighs  10  65  grs.,  and  are  covered  with  silk. 

262.  And,  lastly,  the  measures  are  sometimes  arranged  in  a  form  such  as  this : 

100  > 

in  which  the  large  or  upper  number  represents  the  quantity  passing  the  primary  wire,  the 
under  or  smaller  number  the  quantity  passing  the  secondary  wire,  and  the  decimal  on 
the  right  hand  of  the  bracket,  being  the  quotient  of  the  former  numbers,  is,  as  will 
presently  be  shown,  the  representative  of  the  tension. 

263.  We  have  now  to  examine  the  foregoing  proposition  more  minutely.  Let  us  call 
the  primary  wire,  being  that  which  is  in  connexion  with  the  electromotoric  source,  A; 
and  the  secondary  or  resisting  wire,  B.  Now  how  does  B  act  towards  currents  when 
ihey  are  of  variable  character  ?  There  is  no  current,  no  matter  how  low  its  tension 
may  be,  that  will  not  pass  along  B  to  a  certain  extent:  this  is  abundantly  proved  by 
such  a  wire  transmitting  a  thermal  current  of  the  lowest  tension  and  amount.  But  at  the 
other  extremity  of  the  scale  is  there  a  limiting  point  1  Can  a  wire  conduct  electricity  ot 
a  certain  tension  only  to  a  certain  amount  !  I  think  not,  for  a  wire  of  small  diameter 
was  found  upon  trial  to  conduct  a  thermal  current  to  the  extent  at  one  time  of  20,  and 
then  of  284  parts,  the  tension  in  both  cases  being  the  same ;  and  if  it  would  do  this  in 
the  case  of  currents  whose  tension  is  so  very  low,  the  same  might  be  looked  for  in 
hydro-currents ;  here,  however,  when  the  quantity  reaches  a  certain  point,  the  ignition 
ot  the  wire  ensues,  and  its  physical  character  is  changed.  Sir  Humphrey  Davy's  experi- 
ments lead  to  the  same  conclusion  (Phil.  Mag.,  Dec,  1821),  nor  does  there  appear  to  be 
any  limit  to  the  conducting  power  of  a  wire,  either  for  high  or  for  low  tension.  If  a  wire 
carries  a  certain  amount  of  electricity,  an  increase  of  quantity  or  of  tension  will  enable 
it  to  carry  more,  and  the  converse.    To  this  important  point  I  shall  presently  return. 

264.  As  it  thus  appears  that  any  increase  of  the  quantity  which  A  iransmits  involves 
also  an  increase  of  that  which  passes  B,  a  second  question  arises.  What  is  the  ratio 
that  will  be  observed  in  the  two  cases  ?  If  the  quantity  passing  A  be  doubled,  will  the 
quantity  passing  B  be  doubled  also?  This  is  a  very  important  problem;  for  if  the 
ratio  above  mentioned  holds,  it  would  show  that  an  observation  by  the  secondary  wire 
will  give  the  tension  independent  of  the  absolute  quantity.  Let  a  represent  the  quan- 
tity traversing  A,  and  h  the  quantity  traversing  B.    Now,  if  the  tension  remains  constant, 

and  the  quantity  only  is  variable,  the  ratio  -  is  always  constant,  and  is  entirely  indepen- 

dent  of  the  value  of  «. 

265.  This  I  have  endeavoured  to  prove  experimentally.  I  took  a  hydro-electric  pair 
ot  copper  and  zinc,  each  of  the  plates  exposing  about  two  square  feet  of  surface,  and 
clipped  them  to  different  depths  in  dilute  sulphuric  acid.  The  following  table  exhibits 
one  of  these  results : 


78        RESISTANCE  OF  THE  SECONDARY  WIRE  UNDER  VARIATIONS  OF  TENSION, 


TABLE  A. 


[  Primary  Wire. 

Secon.  Wire. 

Calculated. 

49 

34 

34 

37 

26 

25-6 

24 

17 

16  6 

13 

9-50 

9  0 

and  therefore  we  infer  that  the  foregoing  ratio  holds, 

266.  Currents  of  very  low  tension  give  proofs  of  the  same  fact.  A  thermal  pair  of 
platina  and  palladium  passed  44  through  the  primary,  and  19-50  through  the  secondary 
wire;  and  when,  by  increasing  the  temperature,  236  passed  through  the  primary,  115 
went  through  the  secondary  wire.  In  a  pair  of  palladium  and  silver,  165  and  1130 
being  passed  successively  through  the  primary,  43  and  313  went  through  the  secondary 
wire.  In  a  pair  of  iron  and  platina,  170  and  249  being  successively  sent  through  the 
primary,  79  and  112  respectively  passed  through  the  secondary  wire. 

267.  But  let  us  farther  suppose  that  the  quantity  of  electricity  passing  at  different 
times  through  the  primary  wire  A  is  constant,  its  tension  alone  undergoing  an  increase. 
If  A  formerly  conducted  all  that  was  presented  to  it,  it  will,  under  this  new  condition 
of  things,  of  course  still  do  the  same.    Such,  however,  will  not  be  the  case  with  B,  for 

a  greater  quantity  is  now  enabled  to  pass  it  than  before,  and  the  ratio  -  will  give  a 

CI 

greater  value ;  we  shall  therefore,  in  this  case,  have  a  measure  of  the  tension.  But  if 
the  tension  still  keeps  increasing,  h  will  continually  approach  to  equality  with  a ;  and 
when  the  tension  is  infinitely  high,  these  quantities  are  accurately  equal  to  each  other ; 
or,  in  other  words,  when  the  elastic  force  of  a  current  is  infinitely  high,  its  tension  is 
unity. 

268.  If,  on  the  other  hand,  the  tension  becomes  lower  and  lower,  b  continually  de- 
creases, and,  finally,  might  be  found  equal  to  zero.  The  value  of  the  ratio  then  be- 
comes zero ;  and,  therefore,  at  the  two  extremes,  or  where  the  tension  is  unity  and 
where  it  is  zero,  the  secondary  wire,  so  far  from  ceasing  to  act,  still  truly  indicates  the 
condition  of  the  current. 

269.  While,  therefore,  A  conducts  freely  the  whole  current,  B  will  measure  its  ten- 
sion under  all  circumstances ;  but,  in  point  of  practice,  we  can  never  make  the  adjust- 
ment here  hypothetically  indicated,  or  so  arrange  a  wire  A  that  it  shall  conduct  a/l  the 
electricity  presented  to  it.  Let  us,  therefore,  here  inquire  how  this  variable  condition 
of  both  wires  will  affect  the  result.  Let  the  tension  (l)  so  change  by  any  amount  as 
to  become  f),  then  a  corresponding  change  will  happen  in  a  and  b,  admitting  the 
principle  that  the  quantities  passing  through  A  and  B  are  increasing  functions  of 

If,  then,  (t)  becomes  {n  t),  a  will  become  (n  a).    Now,  if  the  equation  -  —  t  holds, 

CI 

h  =  a  t ;  but,  when  the  change  impressed  on  (J)  has  happened,  b  will  be  equal 
to  the  conjoint  values  of  (n  a)  and  (n  t)  ;  and,  if  these  values  be  substituted  in  the  for- 
mer ratio,  the  result  is  still  equal  to  (n  f)  ;  so  that,  whatever  may  be  the  change  im- 
pressed on  (/),  the  formula  ^  =  t  will  always  indicate  it. 

270.  Having  thus  settled,  by  the  foregoing  simple  reasoning,  the  fundamental  doc- 


CONDITION  OF  THE  CURRENT  NEVER  CHANGES. 


79 


trine  of  investigation,  I  next  proceed  to  apply  it  to  the  analysis  of  the  different  pro- 
cesses, by  which  a  change  of  tension  is  supposed  to  be  impressed  on  an  electric  cur- 
rent; and  this  leads  to  the  consideration  of  the  second  proposition  : 

271.  "  That  there  is  reason  to  doubt  whether  the  processes  usually  supposed  to  af- 
fect the  condition  of  an  electric  current  are  ever  attended  with  any  such  result ;  but 
that,  when  changes  have  apparently  taken  place,  it  is  probable  that  they  may  be  directly 
traced  either  to  a  disturbance  at  the  place  of  generation,  or  to  the  development  of  other 
currents  of  a  different  character,  the  primary  current  itself  remaining  unchanged." 

272.  It  is  popularly  supposed  that  if  we  pass  an  electric  current  through  a  wire  of 
certain  length,  coiled  upon  itself,  a  kind  of  inductive  influence  will  be  exerted,  so  that 
the  current  shall  become  more  and  more  intense  as  it  goes.  Or,  if  two  currents  are 
simultaneously  passed  into  a  double  helix,  they  will  mutually  fortify  each  other. 

273.  (a.)  A  wire,  covered  with  silk,  48  feet  long,  and  arranged  as  one  circular  arc, 
had  a  current  passed  through  it  which  produced  a  deviation  of  35  degrees.  The  same 
wire  was  then  coiled  round  a  piece  of  wood,  so  as  to  make  155  circumvolutions  ;  the 
deviation  was  still  35  ;  and,  therefore,  no  change  was  impressed  on  the  current. 

274.  A  thermal  current  was  passed  through  a  straight  wire  with  the  following 
result : 

2o  |'5238. 

The  wire  was  then  coiled  into  a  helix,  the  current  passed  through  it  and  measured  ;  a 
powerful  bar  magnet  was  next  introduced  into  the  helix,  and  then  a  rod  of  soft  iron.  But 
in  all  these  cases  the  measured  numbers  were  absolutely  the  same  as  before.  There- 
fore there  is  no  change  impressed  on  the  thermal  current,  either  in  relation  to  quantity 
or  tension,  by  making  it  pass  along  a  coiled  wire,  or  by  acting  on  it  with  a  magnet  or 
a  bar  of  soft  iron. 

275.  (c.)  The  same  experiments  were  made  with  a  hydro-electric  current,  and  they 
gave  the  same  results. 

276.  (d.)  The  above-mentioned  (li)  thermal  current  was  passed  along  one  of  the  wires 
of  a  double  helix,  and  through  the  other  wire  a  hydro-current  was  passed,  from  a  single 
pair  of  plates;  but  the  tension  and  quantity  remained  the  same  as  before.  On  sending  a 
current  of  still  greater  intensity,  viz.,  from  a  voltaic  series  of  five  pairs  of  plates,  the  same 
result  was  still  obtained ;  the  hydro-current  had  power  enough  to  decompose  water. 

277.  (e.)  On  altering  the  polar  communications,  and  thereby  changing  the  course  of 
the  current,  no  change  whatever  in  the  primary  current,  either  as  to  quantity  or  ten- 
sion, was  observed. 

278.  It  is  well  known,  that  by  using  a  long  wire  as  a  discharger  of  a  single  pair  of 
plates,  a  spark  will  be  obtained  of  a  much  more  brilliant  character  than  when  the  cur- 
rent passes  through  a  shorter  wire  ;  it  is  upon  this  fact  that  the  flat  spiral  riband  coil  is 
constructed.  Many  electricians  have  supposed  that  the  results  obtained  by  this  beau- 
tiful contrivance  were  partly  due  to  the  inducing  action  of  the  successive  spires,  but 
chiefly  to  a  long  and  easy  conducting  channel  being  open  to  the  current,  which  gath- 
ers momentum  in  its  passage.    I  have  already  shown  that  there  is  no  permanent  action 


80 


TENSION  RISES  WITH  LENGTH  OF  WIRE, 


oi  induction  in  the  case  of  a  coiled  wire,  an  observation  applying  equally  to  an  elon- 
gated helix  and  to  a  flat  spiral.  Let  us  now  determine  whether  the  increased  tension 
is  due  to  momentum. 

279.  A  copper  wire  46  feet  long  and  tV  inch  in  diameter  being  arranged  as  the 
discharger  of  a  single  pair  of  plates,  a  brilliant  spark  was  seen  to  pass  ;  but  with  a  wire 
of  the  same  diameter  and  a  foot  long,  the  spark  was  barely  perceptible.  The  quantity 
and  tension  in  each  case  were  now  determined. 


TABLE 

B. 

Short  wire  .  .  . 

39  ) 
12  ] 

•3076 

Long  wire  .  .  . 

1! 

•6153 

Hence,  by  the  use  of  a  long  wire,  we  greatly  increase  the  tension  of  an  electric  current. 
A  second  experiment,  in  which  a  wire  i,  and  a  third,  in  which  a  wire  i  of  an  inch  in 
diameter,  were  used,  gave  analogous  results.  In  neither  of  these  cases,  however,  did 
the  tension  rise  so  high  as  in  the  former ;  it  was  lower  as  the  diameter  of  the  wire  was 
greater. 

280.  This  increase  of  tension  follows  the  increase  of  the  length  of  the  wire,  as  the 
following  measures  show. 

TABLE  C. 


Current  from  a  single  pair  of  plates 

 ■  a  long  wire  introduced 

  a  second  ditto.,  added 

  third   

  fourth  


Quantity. 


79 
44 
27 
31 
15 


Tension. 


■4177 
•5909 
•7223 
■7619 
■8333 


Thus,  by  successively  increasing  the  aggregate  length  of  the  discharging  wire,  the  ten- 
sion continually  increased,  commencing  at  '4177,  and  finally  becoming  -8333.  Similar 
experiments  with  other  wires  gave  similar  results. 

281.  Now  is  this  remarkable  rise  of  tension  due  to  a  momentum  which  the  current 
acquires  on  the  wire  ?  Or  does  it  arise  from  the  fact,  that  the  wire  acts  simply  as  an 
obstacle,  reacting  thereby  on  the  electromotoric  plates,  the  increase  of  tension  being 
due  to  them,  not  it  1  This  is  easily  determined  ;  for  if  the  rise  of  tension  be  due  to  the 
plates  and  not  to  the  wir  >,  a  short  wire,  slender  enough  to  obstruct  the  current  to  the 
same  extent,  ought  to  act  equally  as  well  as  the  long  wire. 

282.  This  experiment,  the  result  of  which  leads  to  the  true  theory  of  voltaic  combi- 
nations, I  shall  carefully  describe. 

283.  I  took  a  copper  wire  46  feet  long  and  A  inch  in  diameter,  and  found  that  it 
stopped  a  certain  portion  of  the  current  coming  from  a  single  pair  of  plates.  The  mi- 
crometer of  the  balance  was  now  turned,  and  the  needle  brought  accurately  to  zero. 
Then  T  cut  off  from  another  slender  copper  wire  such  a  length  (2  feet  10  inches)  as 
to  obstruct  the  current  to  the  same  extent  as  the  long  wire,  the  needle  being  brought, 
when  it  was  interposed  in  the  path  of  the  current,  to  zero.  The  secondary  coil  was 
now  introduced  ;  it  of  course  stopped  off  a  certain  portion  of  the  current ;  but  the  mi- 
crometer was  again  adjusted,  until  the  needle  was  brought  to  zero.    And  now  the  long 


AND  WITH  DISTANCE  OF  THE  PLATES. 


81 


wire  being  introduced,  and  the  slender  one  taken  away,  the  needle  came  again  to  zero. 
But  I  suppose,  if  the  long  wire  had  impressed  more  tension  on  the  current  than  the 
slender  one,  either  by  momentum  or  otherwise,  more  electricity  should  have  passed  the 
secondary  wire  when  it  was  used,  which  is  not  the  case. 

284.  Again,  I  took  a  copper  wire  242  feet  long  and  tV  inch  in  diameter,  and  adjusted 
to  it  a  fine  iron  wire  as  before  :  the  extremities  of  this  wire  were  tinned;  it  was  12^  inches 
long.  Either  of  these  wires  being  used  as  a  discharger,  brought  the  needle  to  the  same 
point  of  the  scale.  On  using  the  secondary  wire  and  the  long  wire  together,  I  adjusted 
the  needle  accurately  to  zero,  and  then  passing  the  current  through  the  fine  wire  and 
secondary  wire,  it  came  again  to  zero.  And  this  was  repeated  often,  and  so  near  was 
the  adjustment,  that,  when  an  assistant  turned  first  one  and  then  the  other  wire  on,  it 
could  not  be  told  which  was  in  action,  or  whether  the  current  had  come  along  the  long 
or  the  short  wire.  A  long  wire,  therefore,  impresses  no  sort  of  change  on  a  current, 
but  merely  serves  as  an  obstacle ;  for,  in  the  first  case,  we  had  one  wire  sixteen  times 
longer  than  the  other,  and  in  this  we  have  a  wire  more  than  230  longer  than  the  one 
with  which  it  is  compared,  yet  the  tension  has  increased  only  to  the  same  amount  in 
both. 

285.  And  the  same  restdts  were  obtained  by  the  voltameter. 

286.  The  current  that  flows  in  a  simple  closed  voltaic  circle  may  be  resisted  in  two 
ways :  1st,  the  length  of  the  wire  connecting  the  plates  may  be  increased,  as  in  the 
foregoing  experiments ;  2d,  the  connecting  wire  remaining  of  constant  length,  the  dis- 
tance of  the  plates  may  be  increased  :  the  result  is  the  same  in  both  cases,  a  rise  of  tension. 


TABLE  D. 


Ex. 

Distance  of  the  plates  in  inches. 

Quantity. 

Tension. 

1 

  275   

Ill 

•7297 

2 

  4-50   

50 

•7600 

3 

  900   

27 

8888 

So  that,  whether  we  obstruct  the  current  by  lengthening  the  connecting  wire  or  by 
increasing  the  distance  of  the  plates,  the  general  effect  is  the  same,  the  tension  imme- 
diately rises ;  that  increase  of  tension  being  due  to  the  plates  themselves,  and  not  to  the 
channel  of  conduction.    This  brings  us  to  the  third  proposition, 

287.  "  That  there  are  two  different  methods  of  accomplishing  these  disturbances,  and 
thereby  of  raising  the  elastic  force  of  a  current.  1st.  That  tension  may  be  augment- 
ed by  the  sacrifice  of  quantity  ;  Volta's  plan  of  a  reduplicated  series,  and  Henry's  rib- 
and coil  in  its  condition  of  equilibrium,  being  examples.  2d.  By  the  introduction  of 
new  affinities  in  the  exciting  cells ;  batteries  charged  with  nitrosulphuric  acid  or  sul- 
phate of  copper  are  examples." 

288.  A  single  pair  of  plates,  under  the  influence  of  a  long  wire,  or  the  spiral  coil, 
presents  a  remarkable  analogy  to  Volta's  pairs  arranged  in  reduplicated  series.  In  point 
of  fact,  they  may  be  considered  as  scarcely  differing  from  each  other  either  in  mode  of 
action  or  in  effect.  The  study  of  the  single  pair  under  this  condition  reveals  at  once 
the  theory  of  the  voltaic  action. 

289.  If  we  inspect  tables  B,  C,  D,  we  are  at  once  furnished  with  the  fundamental 
fact  which  is  the  basis  of  explanation.    When  we  compare  together  the  tension  and 


82 


RELATION  BETWEEN  QUANTITY  AND  TENSION. 


quantity  of  the  electricity  flowing  in  the  primary  wire,  we  are  struck  with  the  fact,  that 
whenever  tlie  one  has  increased,  the  other  has  diminished.  No  matter  ivhat  the  other 
conditions  may  he,  whether  the  communication  is  juade  by  a  long  wire  or  a  short  one, 
whether  the  plates  are  near  or  far  apart,  whenever  the  quantity  is  dbninished,  the  tension 
increases;  and  lohenever  the  quantity  increases,  the  tension  is  diminished. 

290.  The  remarkable  analogy  of  the  ponderable  elastic  fluids,  which,  when  their  vol- 
ume is  diminished,  or,  in  other  words,  condensation  takes  place,  experience  an  increase 
of  tension  or  elastic  force,  is  here  too  broadly  indicated  to  be  mistaken. 

291.  When  I  first  saw  that  removing  the  plates  to  a  greater  distance  apart  deter- 
mined a  given  rise  in  the  elastic  force  of  the  current,  for  a  time  it  appeared  to  me  that 
Dr.  Faraday's  theory  of  the  tension  being  due  to  the  affinity  of  the  zinc  for  oxygen 
must  certainly  be  incorrect.  A  more  extensive  acquaintance  with  the  facts  has  reversed 
that  opinion.  If  the  tension  be  determined  by  the  affinity  of  the  metal  for  oxygen,  wliich 
must  be  a  constant  force,  how  comes  it  to  pass  that  moving  the  plates  to  a  greater  dis- 
tance apart  can  cause  it  to  increase  ?  This  apparent  paradox,  when  properly  under- 
stood, forms  a  fine  illustration  of  the  truth  of  the  doctrine  advanced  in  the  5th,  7th,  and 
8th  series  of  that  philosopher's  researches.  In  what  follows  I  shall,  therefore,  regard 
those  doctrines  as  established. 

292.  Let  us  take  a  given  pair  of  plates,  and  connect  them  together  by  a  slender  wire. 
We  find  that  the  quantity  that  the  plates  generate  is  diminished,  and  its  tension  is  in- 
creased ;  but  that  this  has  not  happened  either  by  gain  of  momentum  or  inductive 
influence  in  the  channel  of  communication,  and  we  are  compelled  to  refer  the  effect 
to  the  resistance  of  the  wire,  placing  the  plates  and  the  electrolyte  between  them  in  a 
state  of  force.  If  this  be  the  action  of  a  resisting  medium,  we  might  suppose  that  by 
continually  increasing  it  we  should  continually  increase  the  tension,  and  when  it  be- 
came infinitely  great,  the  tension  would  be  so  too.  But  what  is  the  true  action  of  a 
slender  wire,  connecting  in  this  way  a  pair  of  plates  \  A  certain  amount  of  electricity 
passes  along  it,  but  not  the  w)hole  quantity  that  the  plates  could  generate  in  a  given  time ; 
yet  we  cannot  suppose  that  all  that  does  pass  comes  from  the  whole  surface  exposed,  and 
not  from  a  fractional  j^f^f't  thereof  The  water  and  zinc  are  ready  to  generate,  and,  as 
it  were,  attempting  to  drive  a  fresh  quantity  of  electricity  through  the  wire;  and,  accord- 
ingly, as  the  quantity  that  actually  passes  becomes  a  greater  and  greater  portion  of 
what  the  system  actually  tends  to  put  in  motion,  the  tension  becomes  less  and  less. 
The  tension  would  therefore  become  zero  if  the  whole  circle  wires,  plates,  and  elec- 
trolyte could  carry  all  that  the  zinc  and  water  could  generate.  The  limit  prescribed 
to  its  diminution  is  the  conducting  power  of  the  electrolyte,  which  is  the  worst  con- 
ductor of  the  system. 

293.  This  hypothetical  condition,  of  a  tension  ranging  near  zero,  is  most  nearly  ap- 
proximated to  in  a  thermal  pair. 

294.  Suppose,  now,  that  everything  remains  the  same  as  respects  wires,  electrolyte, 
distance  of  plates,  &c.,  except  that  the  dimensions  of  both  plates  are  doubled.  Shall 
we  increase  the  tension  1  No  ;  for  although  the  surface  in  action  is  doubled,  and  the 
absolute  quantity  which  the  system  could  generate  is  doubled,  yet  the  quantity  that 


THEORY  OF  TENSION  OF  THE  VOLTAIC  BATTERY. 


83 


passes  both  the  primary  and  secondary  wire  is  also  doubled  :  the  ratio  -  is  therefore 

the  same  as  before.  For  this  reason,  increasing  the  magnitude  of  the  plates  increases 
the  quantity  only,  and  not  the  tension. 

295.  Under  all  these  circumstances,  therefore,  the  tension  depends  on  the  ratio  of 
the  quantity  that  does  pass  the  combination,  to  the  quantity  that  the  system  tends  to 
put  in  motion. 

296.  Before,  however,  we  can  go  farther  in  the  study  of  these  conditions  of  tension, 
or  attempt  to  show  that  the  arrangement  of  Volta,  and  a  single  pair  under  the  influence 
of  a  long  or  thin  wire,  are,  in  point  of  fact,  alike  in  principle,  it  is  necessary  that  we 
should  understand  the  nature  of  the  different  disturbing  actions  that  may  arise  in  the 
generating  cells  of  the  electromotor. 

296.  I  took  a  zinc  plate  7  inches  long  and  3  wide,  and  a  corresponding  copper  : 
the  surface  of  the  former  was  amalgamated  and  the  latter  brightened.  The  plates  were 
fixed  at  an  immovable  distance  from  each  other,  and  immersed  in  a  jar  containing  34 
ounces  of  water.    Sulphuric  acid  was  then  added  by  half  drachms  successively. 

TABLE  E. 


Exp. 

Quantity  of  acid  in  drachms. 

Quantity. 

Tension. 

1. 

i  

20 

555 

2. 

1   

39 

■436 

3. 

H  

5f> 

■382 

4. 

2   

72 

■305 

5. 

2J  

83 

■277 

6. 

3   

98 

■245 

7. 

3i  

112 

■223 

8. 

4   

121 

■214 

9. 

8  

216 

■130 

298.  Here  we  have  an  exemplification  of  the  converse  of  the  fact  already  so  much 
insisted  on.  As  the  quantity  developed  from  the  same  surface  gradually  became  greater 
and  greater,  the  tension  became  less  and  less,  due  to  the  increased  conducting  powers 
of  the  fluid  medium.  It  is  the  same  effect  that  would  have  been  produced  by  constant- 
ly shortening  the  connecting  wire. 

299.  Such  is  the  action  of  increasing  doses  of  sulphuric  acid ;  let  us  now  see  how 
NITRIC  ACID  will  act.  The  copper  plate  being  repolished,  and  the  zinc  reamalgamated, 
and  everything  else  being  as  at  the  commencement  of  the  former  trial,  the  latter  acid 
was  now  added  to  the  cell  in  the  same  way  that  the  former  had  been  used. 


TABLE  F. 


Exp. 

Quantity  of  acid  in  drachms. 

Quantity. 

Tension. 

1. 

i  

14 

•7143 

2. 

1   

22 

63B3 

3. 

1.^  

36 

5555 

4. 

4   

175 

■2400 

These  measures  are  effected  with  some  difficulty,  as  the  acid  acts  somewhat  irregu- 
larly, and  keeps  the  needle  vibrating. 


84 


THEORY  OF  TENSION  OF  THE  VOLTAIC  BATTERY. 


300.  Muriatic  acid,  under  the  same  conditions  and  circumstances,  being  substi- 
tuted, gave  as  follows : 


TABLE  G. 


Exp. 


1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 
10. 
11. 


Quantity  of  acid  in  drachms. 


i 

1 

n 

2 

2^ 

3 

31 

4 

8 
16 
24 


Quantity. 

Tension. 

10 

■8000 

17 

■6530 

23 

■6087 

29 

•5517 

34 

•5000 

39 

•4872 

44 

■46.04 

49 

•4387 

82 

•3169 

145 

•1938 

203 

•1707 

301.  NiTRosuLPHURic  ACID,  tlic  constitucuts  of  which  were  added  alternately  in 
equal  measures,  was  next  tried. 

TABLE  H. 


Exp. 

Acid. 

Quantity  of  drachms. 

Quantity, 

Tension. 

1. 

Sulphuric. 

1 

Y      .      .      .  . 

23 

•5652 

2. 

Nitric. 

i      .      .      .  . 

53 

-.5094 

3. 

Snip. 

1  .... 

72 

•3888 

4. 

Nitr. 

1  .... 

94 

3510 

5. 

Sulp. 

2      .    .    .  . 

178 

•2360 

6. 

Nitr. 

2      .    .    .  . 

190 

•2026 

302.  Solution  of  sulphate  of  copper  was  next  experimented  with. 

TABLE  I. 


Exp. 

Quantity. 

Tension. 

1. 

33 

•8181 

2. 

74 

6081 

3. 

116 

•4741 

These  measures  were  procured  with  difficulty,  owing  to  the  flocculent  deposit  which 
settled  on  the  zinc,  more  rapidly  as  the  solution  was  made  stronger. 

303.  In  the  general  discussion  of  the  measures  given  by  tables  E,  F,  G,  H,  I,  we  still 
see  the  operation  of  the  same  general  law,  that  the  tension  rapidly  diminishes  as  the 
acid  is  added,  and  that  when  the  same  quantity  of  electricity  developed  from  the  same 
amount  of  surface  by  these  different  acids  is  presented  to  the  secondary  wire,  the  quan- 
tities that  can  pass  that  wire  are  very  different;  and  on  making  use  of  these  different 
agents,  it  would  appear  that  they  can  give  rise  to  currents  from  the  same  metalline  sur- 
face, equal  in  point  of  quantity,  but  differing  greatly  in  point  of  tension,  in  the  follow- 
ing order,  beginning  with  the  most  powerful : 

Sulphate  of  copper.  Muriatic  acid. 

Nitric  acid.  Sulphuric  acid, 

Nitrosulphuric  acid, 

304.  Of  these  bodies,  the  muriatic  acid  acts  probably  in  the  way  that  Dr.  Faraday 
has  pointed  out,  but  the  immediate  cause  of  the  rise  of  tension  in  the  others  is  to  be 
traced  to  the  circumstance  that  they  furnish  oxygen  to  the  nascent  hydrogen  ;  and  if  the 
tension  of  the  ordinary  current  is  made  to  depend  on  the  tendency  of  the  zinc  and  ox- 
ygen to  unite,  it  is  reasonable  to  suppose  that  that  tension  will  increase  if  a  new  affin- 
ity be  introduced,  the  action  of  which  should  correspond  with  and  abet  that  of  the  zinc 
for  oxygen.  This  takes  place  when  nitric  acid  or  an  oxysalt  of  easy  decomposibility 
is  added  to  the  solution.    The  tension  of  the  current  is  not  then  determined  by  the  af 


THEORY  OF  TENSION  OF  THE  VOLTAIC  BATTERY.  35 

finity  of  the  zinc  for  oxygen  only,  but  by  all  the  affinities  that  can  take  place  among  all 
the  bodies  in  the  exciting  cell.  We  are,  therefore,  here  led  to  expand  Dr.  Faraday's 
theory,  and  to  regard  what  follows  as  directly  opposed  to  the  theory  of  contact. 

305.  Upon  these  principles,  in  an  ordinary  arrangement  of  copper,  zinc,  and  sulphuric 
acid,  the  tension  of  the  current  is  determined,  by  the  sum  of  the  affinities  of  zinc  for 
oxygen,  and  hydrogen  for  copper,  diminished  by  the  sum  of  the  affinities  of  copper  for 
oxygen,  oxygen  for  hydrogen,  and  hydrogen  for  zinc. 

30G.  But  as,  under  all  ordinary  circumstances,  the  affinities  of  hydrogen  for  zinc  and 
copper  may  be  neglected,  they  being  exceedingly  small  in  comparison  with  the  others, 
we  may  assume, 

307.  That  the  tension  of  the  current  is  equal  to  the  affinity  of  oxygen  for  zinc,  di- 
minished by  the  sum  of  the  affinities  of  hydrogen  and  copper  respectively  for  oxygen. 

308.  If  now  we  introduce  into  the  exciting  cells  nitric  acid  or  sulphate  of  copper,  the 
affinity  of  the  nascent  hydrogen  for  oxygen  is  satisfied,  and  the  resistance  from  this 
source  is  nearly  exterminated,  and  the  tension  of  the  current  is  then  equal  to  the  dif- 
ference of  the  affinities  of  zinc  and  copper  for  oxygen. 

309.  By  thus  exterminating  the  resistance  arising  from  the  affinity  of  hydrogen  for 
oxygen,  we  succeed  in  raising  the  tension  greatly;  if  next^we  get  rid  of  the  affinity  of 
copper  for  oxygen,  the  tension  ought  to  become  still  higher.  This  may,  in  a  measure, 
be  effected  by  making  use  of  a  plate  of  platina,  as  I  found  experimentally. 

310.  In  all  these  cases,  in  which  the  tension  increases  without  loss  of  quantity,  we 
directly  trace  the  action  to  a  disturbance  in  the  exciting  cells.  In  ordinary  voltaic  ar- 
rangements, the  maximum  tension  is  never  reached,  because  the  affinity  of  zinc  for  ox- 
ygen, which  determines  the  current,  is  counteracted  to  a  certain  extent  by  the  affinity 
of  oxygen  for  hydrogen.  If  we  satisfy  that  affinity,  an  increase  of  tension  is  the  result, 
and  accordingly  as  this  is  more  and  more  nearly  effected,  more  and  more  of  the  hydro- 
gen that  ought  to  be  evolved  disappears.  This  remarkable  disappearance  of  hydrogen 
has  been  heretofore  noticed,  but  the  true  office  it  served  has  not  been  detected.  If  a 
battery  is  charged  with  nitrosulphuric  acid,  the  hydrogen  evolved  is  no  longer  the  equiv- 
alent of  the  zinc  expended  ;  in  point  of  fact,  the  gas  evolved  is  no  longer  hydrogen,  but 
a  mixture  of  hydrogen  and  the  binoxide  of  nitrogen,  as  is  proved  by  its  burning  with  a 
green  flame.  I  took  a  small  pair  of  plates,  the  zinc  being  amalgamated  and  the  platina 
freshly  cleaned,  and  placed  them  in  a  mixture  of  six  ounces  of  water  and  one  drachm 
of  sulphuric  acid,  arranging  an  inverted  tube  over  them,  so  as  to  collect  the  gas  from 
the  platina  plate.  I  determined  by  weighing  the  zinc  plate  how  much  was  expended 
in  evolving  a  given  quantity  of  gas;  and  then  successively  adding  sulphuric  acid  until 
the  total  amount  had  reached  eight  drachms,  it  appeared  that  in  each  instance  it  re- 
quired very  nearly  1*79  grains  of  metal ;  but  on  adding  one  drachm  of  nitric  acid  to 
the  mixture,  the  quantity  expended  rose  at  once  to  2'25  grains,  and  on  adding  a 
second,  to  3-00  grains. 

311.  Therefore,  unless  care  is  taken  that  no  oxidizing  body  is  present,  the  voltame- 
ter will  give  deceptive  results.  This  important  precept  should  be  perpetually  borne 
in  mind  by  those  who  employ  it  in  investigations.    A  few  drops  of  nitric  acid  will  at 


86 


KNOWN  METHODS  OF  INCREASING  THE  TENSION  OF  CURRENTS. 


once  vitiate  its  indications ;  and  there  is  reason  to  suspect  that,  under  certain  circum- 
stances, even  the  dilute  sulphuric  acid  with  which  it  is  charged  may  undergo  partial  de- 
oxidation,  and  the  evolved  hydrogen  indicate  an  amount  of  electricity  less  than  is  actually 
passing. 

312.  We  are,  therefore,  in  possession  of  two  distinct  methods  of  indirectly  increas- 
ing the  tension  of  an  electric  current.  The  first  depends  on  the  reduction  of  quantity  ; 
the  second,  on  satisfying  in  the  exciting  cells  affinities  which  tend  to  antagonize  that 
which  determines  the  current. 

313.  Volta's  plan  of  a  reduplicated  series  unquestionably  acts  upon  the  first  of  these 
principles.  It  is  a  fact  admitted  on  all  hands,  and,  therefore,  into  the  proof  it  is  un- 
necessary now  to  go,  that  the  apparent  quantity  circulating  in  the  whole  battery  is  not 
greater  than  that  which  any  one  of  the  pairs  could  generate.  Dr.  Faraday  has  already 
shown  how  an  enormous  quantity  of  zinc  is  thus  expended,  the  equivalent  of  electricity 
being  entirely  sacrificed  for  the  sake  of  increasing  the  tension.  Let  us  see  what  are 
the  facts  in  the  case.  The  first  pair  of  plates  develops  by  the  oxidation  of  a  portion 
of  its  zinc  a  certain  quantity  of  electricity,  which,  passing  through  the  electrolytic  con- 
ductor, arrives  at  the  second  cell ;  here,  however,  it  is  stopped,  as  a  transit  without  de- 
composition is  impossible,  a  decomposition  which  it  is  unable  to  efi'ect.  Continually 
tending  to  pass,  without  the  passage  actually  taking  place,  it  remains,  as  it  were,  on  the 
surface  of  the  second  zinc  plate,  in  a  condensed  state,  reacting  on  the  electricity  which 
that  plate  is  generating,  compressing  and  being  compressed  by  it,  and,  therefore,  increas- 
ing its  elastic  force.  And  the  same  action  continually  occurs,  and  increases  the  ten- 
sion throughout  the  series. 

314.  A  flat  spiral  coil,  or  a  long  connecting  wire,  obviously  acts  in  the  very  same 
way.  It  opposes  a  resistance  to  the  passage  of  the  current,  and  the  plate  instantly  be- 
comes in  a  forced  state.  We  might  almost  regard  the  electric  fluid  as  existing  upon 
the  surface  of  the  zinc,  exerting  to  the  utmost  its  elastic  force  to  pass  the  barrier,  and 
failing  that,  compressing  the  evolved  fluid  as  fast  as  it  is  generated,  and  being  compressed 
by  it.    This,  also,  is  the  case  in  the  pile  of  Volta. 

315.  Thus  far,  therefore,  the  riband  coil  acts  simply  as  a  long  wire,  and  this  may 
be  regarded  as  its  primary  or  statical  effect.  But,  besides  this,  it  gives  rise  to  an 
action  of  an  entirely  different  character,  which  Professor  Henry  pointed  out  and  ex- 
plained. In  the  act  of  making  and  breaking  contact  in  a  system  of  which  it  forms  a 
part,  Faradian  currents  are  generated  by  its  successive  spirals  ;  these  currents  under  the 
latter  condition,  breaking  contact,  coincide  in  direction  with  the  primary  current  then 
just  ceasing  to  pass.  We  must,  however,  carefully  distinguish  between  these  currents 
and  that  which  induced  them.  In  this  respect  some  philosophers  have  unguardedly 
fallen  into  a  very  remarkable  mistake  ;  it  has  been  supposed  that  when  a  thermo-electric 
current  was  passed  through  this  coil,  and  a  spark  obtained,  the  thermal  light  was  seen ! 
The  case  is  exactly  analogous  to  that  in  which  similar  coils  pass  the  jaws  of  a  horse- 
shoe magnet ;  no  one  supposes  that  the  spark  then  elicited  is  due  to  the  electricity  of 
the  magnet  itself,  but  is  simply  a  manifestation  of  the  induced  current ;  the  very  same 
thing  takes  place  when  the  thermal  current  runs  through  the  spires  of  a  flat  coil.  So 


I 


GENERAL  LAW.  gj 

far  as  I  am  informed,  the  magnetic  spark  and  the  true  thermo-electric  spark  have  never 
yet  been  seen. 

316.  These  observations  are  made  in  order  that  I  may  not  be  misunderstood.  It 
is  not  my  object  to  consider  the  different  arrangements  that  can  generate  a  Faradian 
current,  and,  therefore,  in  this  point  of  view  I  dismiss  the  flat  spiral. 

317.  We  now  come  to  the  fourth  and  last  proposition,  which  is,  "That  the  law 
which  regulates  the  connexion  of  the  diminution  of  quantity  or  condensation  with  the 
increase  of  tension,  is  the  same  as  that  which  regulates  the  analogous  phenomena  of 
ponderable  elastic  lluids." 

318.  I  have  not  hesitated  to  use  the  terms  "  compression,"  "  condensation,"  "  elastic 
force,"  in  reference  to  electricity,  though  I  well  know  such  an  application  is  unusual. 
But  it  has  seemed  to  me  that  a  single  pair  might  almost  be  likened  to  a  steam-engine 
boiler,  from  which  if  you  let  the  steam  escape  by  a  wide  tube,  its  elastic  force  is  less 
and  less,  accordingly  as  the  escape  is  more  free ;  but,  if  you  put  upon  it  a  narrow  tube, 
the  vapour  rushes  with  vehemence  through  it,  reaction  in  a  moment  occurs  in  the 
boiler,  the  elastic  force  increases,  and  the  accumulated  steam  pressing  heavily  on  its 
surface,  the  water  boils  in  a  more  laboured  way :  this  narrow  tube  resembles  Henry's 
coil,  or  a  long  or  slender  wire. 

319.  The  following  table  exhibits  numerical  results  obtained  by  the  aid  of  one  of 
Daniell's  constant  batteries,  the  tension  being  continually  increased  by  the  addition  of 
successively  increasing  lengths  of  wire 


TABLE  K. 


Quantity. 

Calculated. 

Tension. 

Without  any  fB^.f""'"S°.f 
wire  inter  J     t'-e  expen- 

,        1  ment, 
P""^''-        i  End  of  ditto. 
Wire  interposed  6  feet  long 
12 

<c         «  18 
"         "  24 
48 

"         "        72  " 
"         "  96 

j.76 

75 
72 

68.50 

65 

62 

53 

46 

41 

7200 
68  .30 
65-24 
62  30 
53  12 
46  18 
41  00 

■8289 

•8333 
•8394 
•8461 
•8548 
•8679 
•8913 
-9219 

320.  From  this  table  it  would  appear  that  the  addition  of  successively  increasing 
lengths  of  wire  of  invariable  diameter  diminishes  the  absolute  quantity  of  electricity 
flowing,  but  at  the  same  time  the  tension  is  exalted.  By  taking  the  angle  of  torsion  as 
the  measure  of  the  forces,  in  the  second  column,  it  is  also  evident  that  the  law  of  the 
conducting  power  of  wires  given  by  M.  Lenz  holds  in  the  case  of  a  hydro-electric  pair. 
This  may  be  regarded  as  of  some  interest,  inasmuch  as  the  late  Dr.  Ritchie,  in  certain 
papers  read  before  the  Royal  Society,  opposed  to  the  very  last  this  view,  by  the  aid 
of  numerical  determinations  made  with  the  torsion  balance,  the  instrument  here  em- 
ployed. In  reference  to  the  third  column  of  the  table,  I  have  calculated  it  in  the  man- 
ner given  by  Lenz,  the  value  of  the  constant  to  be  deduced  from  the  reciprocals  of  the 
angles  of  torsion  being  in  this  case  1318  nearly. 

321.  While,  therefore,  these  results  confirm,  in  the  most  pointed  manner,  the  reason- 
ing of  that  able  philosopher,  they  at  the  same  time  compel  us  to  advance  a  step  far- 


88 


CASE  OF  THERMO-ELECTRICITY  AND  MACHINE  ELECTRICITY. 


ther,  and  to  expand,  to  a  certain  extent,  Ohm's  theory  of  the  voltaic  pile.  It  is  a  con- 
dition, in  tracing  the  action  of  wires  of  different  lengths,  to  assume  that  the  electromo- 
tive power  of  the  generating  pair  is  under  all  circumstances  constant,  and  hence  it  may 
be  conveniently  represented  by  unity.  But  the  electromotive  power  of  any  pair  plainly 
depends  on  tivo  things:  the  quantity  of  electricity  that  the  pair  can  evolve,  and  its  abso- 
lute tension.  The  theory  of  Ohm,  as  may  be  gathered  from  the  memoir  of  Professor 
Jacob:  on  electromotive  machines,  and  also  from  M.  Lenz's  papers,  confounds  those 
two  important  conditions. 

322.  Now  the  results  given  in  the  foregoing  table,  proving  that  wires  conduct  in 
the  inverse  ratio  of  their  lengths,  prove,  also,  that  the  addition  of  increasing  lengths  of 
wire  does  not  in  any  wise  alter  the  electromotive  power ;  yet  we  have  clearly  shown 
that  this  addition  is  inevitably  attended  with  an  increase  of  tension.  Here,  therefore, 
is  an  apparent  contradiction. 

323.  But  this  contradiction  is  only  apparent,  and,  when  properly  understood,  leads  to 
a  most  remarkable  result. 

324.  It  is  true  that  we  are  compelled  to  assume  that  the  electro-motive  power  of  a 
pair  is  independent  of  the  length  of  the  connecting  wire  ;  but  this  constancy  of  electro- 
motive power  does  not  necessarily  imply  that  the  relations  of  quantity  and  tension, 
which  conjointly  produce  it,  are  not  themselves  variable.  In  the  case  before  us,  we 
have  direct  proof  that  the  tension  increases,  and  also  that  the  quantity  decreases,  as 
the  connecting  wire  becomes  longer,  and  the  converse  ;  yet  the  electromotive  power 
varying  directly  with  them  both,  they  must  of  necessity  bear  such  a  relation  to  each 
other,  that  their  product  shall  always  be  equal  to  unity.    Hence  we  infer, 

325.  That  the  law  of  Marriotte  in  relation  to  the  ponderable  elastic  fluids  holds, 
also,  in  the  case  of  electricity  developed  by  voltaic  action,  the  elastic  force  or  tension 
of  a  given  quantity  being  inversely  as  the  space  it  occupies. 

326.  The  following  table  will  at  the  same  time  establish  Lenz's  law  in  the  case  of 
thermo-electric  currents,  and  prove  that  even  in  cases  where  the  tension  is  so  exceedingly 
low,  the  elastic  force  of  a  given  quantity  of  electricity  follows  the  above-named  law. 


TABLE  L. 


Quantity. 

Calculated. 

Tension. 

Without  any  wire    .    .  . 

720 

•0930 

Interposed  wire  1  foot  long 

298 

298 

•2080 

"         "    2  feet  long 

192 

192 

•3021 

"          "  .3 

142 

142 

■3802 

"  4 

112 

113 

■4375 

«  5 

93 

93 

•4731 

"  6 

80 

80 

■5000 

The  current  here  experimented  with  was  generated  by  a  pair  of  wires  of  copper  and 
tinned  iron  tV  inch  in  diameter,  and  one  foot  long,  the  soldered  extremity  being  im- 
mersed in  a  bath  of  l)oiling  water,  and  the  free  extremity  carefully  maintained  at  59i 
Fahr. ;  the  third  column  in  the  table  being  calculated  by  the  aid  of  the  constant  1527. 

327.  As  respects  electricity  of  high  tension,  a  law  extremely  analogous  to  that  here 
indicated  may  be  traced.  The  striking  distance  varies  directly  as  the  quantity  accu- 
mulates. If  a  given  jar  be  successively  charged  with  quantities  of  electricity,  as  the 
numbers  1,  2,  3,  4,  &c.,  the  intervals  of  air  through  which  the  spark  can  pass  vary  di- 


CASE  OF  THERMO-ELECTEICITY  AND  MACHINE  ELECTRICITY. 


89 


rectly  as  those  numbers.  This  is  abundantly  shown  by  the  experiments  of  Lane,  Har- 
ris, and  other  philosophers. 

328.  Now  upon  what  does  this  striking  distance  depend  1  Plainly  upon  the  elastic 
force  of  the  coerced  fluid,  and  therefore  the  striking  distance  will  measure  the  elastic 
force  or  tension.  We  condense  upon  a  given  surface  increasing  quantities  of  the  elec- 
tric fluid,  and  find  that  the  law  in  relation  to  its  elastic  force  is,  that  the  tension  of  a 
given  quantity  is  inversely  as  its  volume.  But  this  is  the  law  of  Marriotte  in  relation 
to  the  ponderable  elastic  fluids. 

329.  The  following  numerical  determinations  were  made  by  adding  successive  plates 
to  the  first  single  hydro  pair,  and  taking  the  values  of  the  current  on  each  addition.  It 
is  offered  merely  as  an  illustration  of  the  chief  fact  under  discussion,  and  is  not  to  be 
regarded  as  absolutely  correct,  though  every  precaution  was  taken  to  avoid  changes  in 
the  current.  It  shows  the  decrease  of  quantity  and  the  increase  of  tension  in  Volta's 
instrument.  Of  course,  in  reasoning  upon  it,  the  hypothetical  action  of  each  plate  is 
assumed  to  be  equal  to  that  of  any  other  in  the  series. 


TABLE  M. 


No.  of  Plates. 

Quantity. 

Tension. 

1. 

20 

•3500 

2. 

31 

•5806 

3. 

43 

■5814 

4. 

51 

•6274 

5. 

52 

•6346 

330.  Thermo-electric  piles  are  well  known  to  give  the  same  general  results,  as  respects 
tension,  that  hydro-electric  piles  do  ;  they  are  much  better  suited  to  the  purpose  of  the 
experimenter,  and  give  currents  that  are  far  more  constant.  The  following  table  rep- 
resents the  action  of  such  a  battery,  consisting  of  wires  of  copper  and  tinned  iron,  each 
element  being  about  one  foot  long  and  inch  in  diameter.  The  source  of  heat  was 
a  bath  of  boiling  water. 


TABLE  N. 


No.  of  Pairs. 

Quantity  |  Tension. 

1. 

•256 

1367 

2. 

305 

2065 

3. 

.325 

•2707 

4. 

348 

3072 

5. 

3.52 

■3204 

11. 

396 

•5275 

331.  The  beautiful  experiments  of  Becquerel,  and  the  equally  elegant  repetition  of 
them  by  Dr.  Golding  Bird,  show  that  the  view  I  have  here  taken  of  the  action  of  a  sin- 
gle pair  is  correct.  The  latter  chemist  found,  that  not  only  could  a  single  pair  decom- 
pose bodies,  such  as  iodide  of  potassium,  &c.,  which  easily  yield  up  their  elements,  but 
that  the  ammoniacal  amalgam  might  be  formed,  potassium  reduced,  and,  in  point  of  fact, 
any  decomposition  effected.  And  wbat  is  the  plan  followed  1  The  current  is  forced  to 
pass,  in  the  electrolyte  that  is  to  be  decomposed,  an  obstacle  or  resisting  medium;  the  ten- 
sion instantly  rises,  but  at  a  vast  sacrifice  of  quantity,  so  that  the  magnetic  needle,  which 
measures  only  the  quantity  passing  in  an  indivisible  portion  of  time,  is  barely  affected. 
Yet,  by  continuing  the  current  for  a  great  length  of  time,  the  resulting  decomposing 
effects  are  finally  the  same  as  those  obtained  more  speedily  by  the  action  of  many  pairs. 

M 


90 


ELECTRIC  SPARK  BEFORE  CONTACT  IN  VACUO. 


332.  How  far  the  experiments  given  in  this  memoir  bear  upon  that  part  of  Dr.  Fara- 
day's researclies,  in  which  he  has  determined  the  relation  of  common  and  vohaic  electricity 
by  measure,  would  form  a  most  important  subject  of  investigation.  The  results  at  which 
he  arrives  are  in  themselves  very  astonishing,  and  are  fully  borne  out  by  his  decisive 
experiments ;  but  when  we  come  to  reflect  that  these  results  were  obtained  by  the  mag- 
netic needle  and  electro-chemical  action,  we  may,  perhaps,  pause.  We  may  ask  whether 
it  is  possible  to  determine  by  either  of  these  means  the  absolute  quantity  of  electricity 
that  passes.  Both  measure,  so  to  speak,  the  volume  that  flows,  the  one  in  an  indivisi- 
ble portion  of  time,  the  other  that  which  has  flowed  at  the  end  of  a  finite  time ;  but  do 
either  of  them  measure  the  true  absolute  quantity  ?  Can  we  tell  the  absolute  amount 
of  a  gas  without  first  knowing  its  condition  as  to  condensation  ?  Can  we  know  liow 
much  electricity  is  upon  a  prime  conductor,  or  compare  it  with  that  evolved  by  a  vol- 
taic pile,  without  first  knowing  its  state  of  condensation  \  I  shall  be  excused  for  em- 
ploying these  expressions  in  an  unusual  way,  and  for  reasoning  about  this  subtle  agent 
as  though  it  were  a  ponderable  body,  inasmuch  as  it  serves,  without  introducing  any 
hypothesis,  to  give  us  more  tangible  and  distinct  ideas  of  what  we  might  otherwise 
vainly  attempt  to  express. 

333.  In  the  December  number  of  this  journal  (i.  and  E.  Phil.  Mag.,  vol  xiii.,  p.  401), 
which  has  just  reached  me,  I  find  some  remarks  of  Dr.  Jacobi  on  the  galvanic  spark. 
Some  time  ago  I  came,  by  another  method  of  experimenting,  to  the  same  conclusion. 
If  this  spark  be  really  projected  by  the  tension  before  contact,  it  ought  to  take  effect  at 
an  unlimited  distance  in  a  perfect  vacuum ;  but  it  will  be  found,  on  making  the  trial, 
that  if  an  iron  electrode  be  sealed  into  the  upper  part  of  a  barometer  tube,  and  the  mer- 
cury made  to  rise  gradually  towards  its  point,  the  spark  does  not  pass  until  apparent 
contact  takes  place  :  this  was  found  in  an  analogous,  but  vain  attempt  to  show  the  ther- 
mo-electric spark.  It  cannot,  however,  be  entirely,  as  that  philosopher  supposes,  "sim- 
ply a  phenomenon  of  combustion,"  as  it  is  difficult  to  understand  how  mercury  can  enter 
into  combustion  in  a  vacuum. 


CHAPTER  IX 

ON  THE  ELECTROMOTIVE  POWER  OF  HEAT. 

{From  the  London  and  Edinburgh  Philosophical  Magazine  for  June,  1840.) 

Contents  :  Object  of  the  Memoir. — Experimental  Arrangement  to  determine  the  Elec- 
tromotive Power. —  Temperatures  calculated  from  Quantities  of  Electricity. — Increase 
of  Tension  with  increase  of  Temperatiire. — Depends  on  increased  Resistance  to  Con- 
duction.— Quantity  of  Electricity  independent  of  heated  Surface. — In  Thermo-electric 
Piles,  the  Quantity  of  Electricity  propoi'tional  to  the  Number  of  Pairs. — Best  Forms 
of  Construction  of  Thermo-electric  Pairs. 

334.  From  the  memoir  of  M.  Melloni,  on  the  Polarization  of  Heat,  inserted  in  the 
second  part  of  the  first  volume  of  the  Scientific  Memoirs,  we  learn  that  M.  Becquerel, 


EXPERIMENTAL  ARRANGEMENT  TO  DETERMINE  THE  ELECTROMOTIVE  POWER.  91 

as  well  as  himself,  has  made  experiments  to  determine  the  quantities  of  electricity  set  in 
motion  by  known  increments  of  heat.  From  these  experiments,  they  conclude  that 
through  the  whole  range  of  the  thermometric  scale,  those  quantities  are  directly  propor- 
tional to  each  other. 

335.  But  as  thermo-electric  currents  are  now  employed  in  a  variety  of  delicate  physi- 
cal investigations,  and  as  there  appears  to  be  much  misconception  as  to  their  character,  I 
propose  in  this  memoir  to  show, 

1st.  That  equal  increments  of  heat  do  not  set  in  motion  equal  quantities  of  electricity. 

2dly.  That  the  tension  undergoes  a  slight  increase  with  increase  of  temperature;  a 
phenomenon  due  to  the  increased  resistance  to  conduction  of  metals,  when  their  tem- 
perature rises. 

3dly.  That  the  quantity  of  electricity  evolved  at  any  given  temperature  is  independ- 
ent of  the  amount  of  heated  surface  ;  a  mere  point  being  just  as  efficacious  as  an  indefi- 
nitely extended  surface. 

4thly.  That  the  quantities  of  electricity  evolved  in  a  pile  of  pairs  are  directly  propor- 
tional to  the  number  of  the  elements. 

336.  First,  then,  as  to  the  comparative  march  of  electric  development,  with  the  rise 
of  temperature,  in  the  case  of  pairs  of  different  metals. 

337.  The  experimental  arrangement  which  I  have  employed  is  represented  in  fg.  31. 
A  A  is  a  glass  vessel  about  three  inches  in  diameter,  with  a  wide  neck,  through  which 
can  be  inserted  a  mercurial  thermometer,  h,  and  one  extremity  of  a  pair  of  electro-motoric 
wires.  The  wires  I  have  employed  have  generally  been  a  foot  long,  and  one  sixteenth 
of  an  inch  in  diameter.  The  extremity,  .f,  of  the  wires  thus  introduced  into  the  vessel 
ought  to  be  soldered  with  hard  solder ;  their  free  extremities  dip  into  the  glass  cups,  d  d, 
filled  with  mercury,  and  inmiersed  in  a  trough,  e,  containing  water  and  pounded  ice.  By 
means  of  the  copper  wives,  j]/',  one  sixth  of  an  inch  thick,  communication  is  established 
with  the  mercury  cups  of  the  galvanometer.  The  coil  of  this  galvanometer  is  of  cop- 
per wire  one  eighth  of  an  inch  thick,  and  making  twelve  turns  only  round  the  needles, 
which  are  astatic.  The  deviations  were  determined  by  the  torsion  of  a  glass  thread,  in 
the  way  described  in  Chapter  VIII. 

338.  It  is  surprising  to  those  who  have  never  before  seen  the  experiment,  with  what 
promptitude  and  accuracy  a  copper  and  iron  wire,  soldered  thus  together,  will  indicate 
temperatures. 

339.  In  the  arrangement  now  described,  when  an  experiment  has  to  be  made,  the 
vessel  A  A  is  to  be  filled  two  thirds  full  of  water,  the  bulb  of  the  thermometer  being 
so  adjusted  as  to  be  in  the  middle  of  the  vessel,  and  the  soldered  extremity,  s,  of  the  two 
wires  being  placed  in  contact*  with  it,  and  a  small  cover  with  suitable  apertures  ad- 
justed on  the  top  of  the  vessel,  so  that  the  steam,  as  it  is  generated,  may  rush  up  along- 
side of  the  tube  of  the  thermometer,  and  bring  the  mercurial  column  in  it  to  a  uniform 
temperature.  The  communicating  wires,  ff,  are  then  placed  in  the  cups,  and  the  trough, 
e,  filled  with  water  and  pounded  ice,  and  carefully  surrounded  with  a  flannel  cloth.  The 


*  If  the  extremity  of  the  thermo-electric  pair  be  allowed  to  rest  on  the  bottom  of  the  glass  vessel,  no  accurate  results  can 
be  obtained ;  the  pair  does  not  then  indicate  the  temperature  of  the  water. 


92 


TEMPERATURES  CALCULATED  FROM  QUANTITIES  OF  ELECTRICITY. 


water  in  the  vessel  A  A  is  then  gradually  raised  to  the  boiling  point  by  means  of  a 
spirit-lamp,  and  kept  at  that  temperature  until  the  galvanometer  needles  and  the  ther- 
mometer are  quite  steady.  The  same  plan  must  be  followed  when  any  other  tempera- 
ture than  212  is  under  trial,  for  the  thermo-electric  wires  changing  their  temperature 
more  rapidly  than  the  mercury  in  the  thermometer,  it  is  absolutely  necessary  to  continue 
the  experiment  for  some  minutes  to  bring  both  to  the  same  state  of  equilibrium. 

340.  When  a  temperature  higher  than  212°  Fah.,  but  under  a  red  heat,  is  required, 
I  substitute,  in  place  of  the  vessel  A  A,  a  tubulated  retort,  the  tubulure  of  which  is 
large  enough  to  allow  the  passage  of  the  bulb  of  the  thermometer  and  the  wires.  A 
quantity  of  mercury  sufficient  to  fill  the  retort  half  full  is  then  introduced,  and  the  tubu- 
lure being  closed  by  appropriate  pieces  of  soapstone,  the  neck  of  the  retort  is  inclined 
upward,  so  that  the  vapour  as  it  rises  may  condense  and  drop  back  again  without  in- 
commoding the  operator.  As  in  the  former  case,  it  is  here  also  necessary  to  continue 
each  experiment  for  a  few  minutes,  to  bring  the  thermometer  and  the  thermal  pair  to 
the  same  condition.  There  is  not  much  difficulty  in  obtaining  any  required  tempera- 
ture, by  raising  or  lowering  the  wick  of  the  lamp. 

341.  The  metals  I  have  tried  were  in  the  form  of  wires.  They  were  in  the  state 
found  in  commerce,  and  therefore  not  pure  ;  they  were  obtained  in  the  shops  of  Phil- 
adelphia 

TABLE  L 


Names  of  the  pairs  of  Metals. 


Copper  and  iron  .  . 
Silver  and  palladium. 
Iron  and  palladium 
Platina  and  copper  . 
Iron  and  silver  .  . 
Iron  and  platina   .  . 


Temperatures  (Fahr.). 


32  F. 


122  F. 


93 
65 
112 
11 

89 
28 


212  F. 


176 
147 
223 

26 
137 

56 


662  F. 


233 
613 
631 

122  f  t- 
244 

248. 


In  this  table  I  have  estimated  the  temperature  of  boiling  mercury  at  662°  Fahr. 
The  quantities  of  electricity  evolved,  as  estimated  by  the  torsion  of  a  glass  thread,  are 
ranged  in  columns  under  their  corresponding  temperatures.  Each  series  of  numbers  is 
the  mean  of  four  trials,  the  differences  of  which  were  often  imperceptible,  and  hardly 
ever  amounted  to  more  than  one  degree. 

342.  Now  if  this  table  be  constructed,  the  temperatures  being  ranged  along  the  axis 
of  abscissas,  and  the  quantities  of  electricity  being  represented  by  corresponding  ordi- 
nates,  we  shall  have  results  similar  to  those  given  in  Jig.  32,  in  which  it  is  to  be  ob- 
served that  the  curves  given  by  the  systems  of  silver  and  iron,  copper  and  iron,  and  pal- 
ladium and  iron,  arc  concave  to  the  axis  of  abscissas  ;  but  those  given  by  platina  and 
copper,  silver  and  palladium,  and  platina  and  iron,  are  convex. 

343.  Let  us  now  apply  the  numbers  obtained  by  these  several  pairs  for  the  calcula- 
tion of  temperatures,  which  will  set  their  action  in  a  more  striking  point  of  view.  The 
following  table  contains  such  a  calculation,  on  the  supposition  that  for  the  90  degrees 
from  32°  Fahr.  to  122°  Fahr.,  the  increments  of  electricity  are  proportional  to  the  tem- 
peratures. 


/ 

TEMPERATURES  CALCULATED  FROM  QUANTITIES  OF  ELECTRICITY. 

TABLE  II. 


Tcmpenitures  by  the  Mercurial  Thermometer. 

32  F. 

122  F. 

Water  boils. 

Mercury  boils. 

f  Copper  and  iron     .  . 

32 

122 

202 

257 

1  Silver  and  palladium  . 

32 

122 

235 

880 

2  Z 

J  Iron  and  palladium 

32 

122 

211 

539 

j  Platina  and  copper 

32 

122 

244 

1030 

1" 

1  Iron  and  silver  .    .  . 

32 

122 

170 

279 

Iron  and  platina 

32 

122 

212 

829 

344.  We  are  therefore  led  to  the  general  conclusion  that,  in  these  six  (liferent  sys- 
tems of  metals,  the  developments  of  electricity  do  not  increase  proportionally  with  the 
temperatures,  hut  in  some  with  greater  rapidity,  and  in  others  with  less. 

345.  The  results  here  given  I  have  corroborated  in  a  variety  of  ways,  and  with  a 
variety  of  wires.  A  pair,  consisting  of  copper  and  platina,  gave  for  the  temperature  of  tin, 
when  in  the  act  of  congealing,  452°  Fahr.  instead  of  442°  Fahr.,  the  point  usually  taken. 
For  the  melting  point  of  lead,  it  gave  942j°  Fahr.,  instead  of  612°  Fahr.  The  melt- 
ing points  of  tin,  lead,  zinc,  and  occasionally  of  antimony  and  bismuth,  were  in  this 
manner  employed,  for  they  allow  time  for  the  working  of  the  torsion  balance,  and,  with 
the  exception  of  bismuth,  their  temperature  appears  to  be  steady  all  the  while  they  are 
in  a  granular  condition,  before  they  finally  solidify.  The  action  of  these  metals  on  the 
thermo-electric  pair  is  easily  prevented  by  dipping  it  into  a  cream  of  pipe-clay. 

346.  A  pair  of  copper  and  platina  gave  for  a  dull  red  heat  1416°  Fahr.,  and  for  a 
bright  red  2103"  Fahr. 

347.  A  pair  of  palladium  and  platina  gave  for  a  dull  red  1850°  Fahr.,  and  for  a  bright 
red  2923°  Fahr. 

348.  Some  of  the  combinations  into  which  iron  enters  as  an  element  give  rise  to 
remarkable  results  ;  thus,  if  we  project  the  curve  given  by  a  system  of  copper  and  iron, 
we  shall  find  it  resembling^"-.  33,  where  the  maximum  ordinate  h  occurs  at  a  tempera- 
ture of  about  650°  Fahr. ;  the  point  c  appears  to  be  given  between  700  and  800  degrees  ; 

by  a  dull  red  heat ;  e  is  very  nearly  the  point  at  which  an  alloy  of  equal  parts  of  brass 
and  silver  melts,  for  if  the  pair  be  soldered  with  this  substance,  it  fuses  when  the  needles 
have  returned  almost  exactly  to  the  zero  point.  With  harder  solders,  or  with  wires 
simply  twisted,  the  curve  may  be  traced  on  the  opposite  side  of  the  axis  towards its 
ordinate  increasing  with  regularity.  At  60°  Fahr.,  taking  the  length  of  the  ordinate 
corresponding  to  a  temperature  of  212"  Fahr.  as  unity,  the  length  of  the  maximum  or- 
dinate at  b  is  1-85,  very  nearly. 

349.  A  system  of  silver  and  iron  gives  also  a  similar  curve,  the  point  h  occurring  at 
a  temperature  rather  higher  than  the  analogous  one  for  the  preceding  system,  but  still 
below  the  boiling  point  of  mercury. 

350.  Now  all  these  things  serve  to  show  that  we  cannot  determine  with  accuracy 
unknown  temperatures  by  the  aid  of  thermo-electric  currents,  on  the  supposition  that 
the  increments  of  the  quantities  of  electricity  are  proportional  to  the  increments  of 
temperature  throughout  the  range  of  the  mercurial  thermometer. 

351.  Let  us  now  proceed  to  the  second  proposition,  "  That  the  tension  undergoes  a 
slight  increase  with  increase  of  temperature,  a  phenomenon  due  to  the  increased  resist- 
ance to  conduction  of  metals  when  their  temperature  rises." 


94 


INCREASE  OF  TENSION  WITH  INCREASE  OF  TEMPERATURE. 


352.  It  will  be  seen,  on  consulting  the  following  table,  that  pairs  of  different  metals, 
at  the  same  temperature,  have  tensions  which  are  apparently  very  different. 

353.  The  currents,  the  tensions  of  which  are  here  indicated,  were  generated  by  keep- 
ing one  end  of  the  thermal  pair  in  boiUng  water,  the  other  ends  being  maintained  at  a 
temperature  of  32°  Fah. 

TABLE  III. 


A  pair  of 

Tf  nsion. 

A  pair  of 

Tension. 

Antimony  and  bismuth  . 

■137 

■470 

Copper  and  iron  

■183 

Copper  and  platina  .  .  . 

■473 

Silver  and  lead   

■307 

Platina  and  palladium  . 

500 

Lead  and  palladium  .  .  . 

■313 

Tin  and  iron  

•518 

Silver  and  platina  .... 

■380 

Platina  and  tin  

■567 

We  perceive,  therefore,  that  there  apparently  exist  specific  differences  in  the  qualities 
of  electric  currents  derived  from  different  sources.  If,  for  example,  we  take  a  pair  of 
platina  and  palladium,  and  expose  it  to  a  temperature  which  shall  generate  a  current 
capable  of  deflecting  the  torsion  balance  through  1000  degrees,  and  then  obstruct  it  by 
a  wire  of  such  dimensions  as  to  stop  one  half,  or  only  allow  500  degrees  to  pass,  and 
repeat  the  experiment  with  a  current  generated  by  bismuth  and  antimony,  the  tempera- 
ture being  still  so  adjusted  as  to  give  a  deflection  of  1000  degrees,  on  making  this  pass 
through  the  same  intercepting  wire,  perhaps  not  much  more  than  one  eighth  of  it  will 
go  through  the  galvanometer. 

354.  It  might  be  supposed  that  these  characteristic  differences  of  thermal  currents, 
derived  from  different  sources,  were  due  to  some  modification  of  the  electricity  itself, 
similar  to  those  of  radiant  heat  derived  from  different  sources,  or  at  different  tempera- 
tures, which  M.  Melloni  has  attempted  to  show  are  analogous  to  the  colours  of  light, 
being,  like  them,  of  different  degrees  of  refrangibility,  and  permeating  absorbent  media 
with  different  degrees  of  facility.  For  in  the  same  way  that  we  regard  glass  as  trans- 
parent to  light,  and  rock  salt  as  transparent  to  heat,  so,  too,  we  might  regard  a  copper 
wire  or  any  conducting  medium  as  transparent  to  electricity. 

355.  But  this  peculiarity  of  thermo-electric  currents  depends  on  the  conducting 
resistance  of  the  system  that  generates  them.  It  is  possible  to  give  a  current  a  higher 
or  a  lower  tension,  by  simply  making  use  of  thin  or  thick  wires  to  generate  or  to  carry 
it.  In  the  foregoing  table,  the  current  from  platina  and  palladium  had  a  high  tension, 
because  slender  wires  of  those  metals  happened  to  be  used  to  generate  it ;  and  the  cur- 
rent from  antimony  and  bismuth  had  a  low  tension,  because  thick  bars  of  those  sub- 
stances were  employed.  In  the  former  case,  the  conducting  resistance  was  greater  than 
in  the  latter,  and  hence  the  tension  of  the  current  was  higher. 

356.  That  this  is  strictly  true  will  appear  on  examining  the  current  evolved  by  any 
number  of  systems  under  the  same  condition  of  resistance  to  condition.  I  took  a  pair 
of  copper  and  iron,  and  soldered  it  to  a  simple  pair  of  platina  and  copper,  as  is  shown 
in  fig.  34,  so  as  to  form  one  continuous  metallic  fine.  The  point  of  junction  formed 
by  the  wires  i  (iron)  and  p  (platina)  was  kept  carefully  at  63°  Fah.,  by  immersion  in 
a  water-bath  ;  the  point  of  junction,  p  (platina)  and  c  (copper),  was  treated  in  like 
manner,  but  that  of  e  and  i  was  raised  to  212°  Fah.  Under  these  circumstances,  it  was 
found  that  181  degrees  of  electricity  were  evolved,  of  which  fifty  went  through  a  given 


DEPENDS  ON  INCREASED  RESISTANCE  TO  CONDUCTION.  95 

secondary  wire.  Then,  raising  the  junction  p  and  c  to  212"  Fah.,  and  bringing  e  and  i 
to  63"  Fah.,  there  passed  at  the  galvanometer  71  degrees,  of  which  19  could  traverse  the 
same  secondary  wire,  but 

As  181  :  50  :  :  71  :  19-6; 
and  hence  I  infer  that,  where  the  conducting  resistance  is  the  same,  the  tension  of  cur- 
rents from  different  sources  does  not  differ. 

357.  These  results  inform  us  how  much  the  tension  of  a  current  depends  on  the  re- 
sistance to  conduction  of  the  system  which  it  traverses,  as  well  as  on  the  dimensions 
of  the  system  itself;  an  observation  the  value  of  which  we  shall  presently  see. 

358.  In  a  great  number  of  trials  which  I  made,  I  failed  in  getting  any  *nistworthy 
results  as  respects  tension  of  currents  at  high  temperatures,  on  account  of  the  difficulty 
of  maintaining  the  thermo-electric  pair  at  the  same  degree  without  variation.  By  em- 
ploying, however,  a  small  blacklead  furnace,  to  which  was  adapted  a  covered  sand- 
bath,  into  which  the  wires  could  be  plunged,  I  succeeded  at  last ;  for  with  this  arrange- 
ment a  regulated  temperature  could  be  kept  up  for  a  length  of  time. 

359.  The  experiment  was  made  with  care  in  the  case  of  two  systems  of  metals: 
1st,  copper  and  platina ;  2d,  copper  and  iron. 

1st.  At  the  boiling  point  of  water,  a  pair  of  copper  and  platina,  the  unexcited  ex- 
tremity of  which  was  carefully  maintained  at  67°  Fah.,  evolved  as  a  mean  of  four  trials, 
three  of  which  were  absolutely  identical,  123  degrees  of  electricity,  of  which  23  could 
pass  a  secondary  wire. 

Then,  by  the  aid  of  the  furnace  and  sand-bath,  the  temperature  was  raised  until  the  pair 
evolved  783  degrees  as  a  mean  of  four  trials ;  of  these  163  could  pass  the  secondary  wire. 
Now,  As  783  :  163  :  :  123  :  25^  instead  of  23, 

showing,  therefore,  a  slight  rise  of  tension. 

300.  2d.  The  pair  of  copper  and  iro7i  gave,  at  the  boiling  point  of  water,  300  degrees, 
of  which  57  passed  the  secondary  wire.  The  temperature  was  now  raised,  with  the 
following  results : 

490  degrees  passmg  the  primary,  95  the  secondary  wire. 

553      "  "  "  113 

545      "  "  "      112  " 

493      "  "  "  110 

It  will  be  understood,  that  although  the  quantities  of  electricity  indicated  in  the  first 
column  do  not  regularly  increase,  the  temperatures  w-ere,  notwithstanding,  going  regu- 
larly upward  ;  to  this  peculiarity  of  the  systems  into  which  iron  enters  I  have  already 
alluded  (348).  Let  us  now  compare  these  measures  with  those  obtained  for  the  boiling 
point  of  water : 

As  490  :    95  : :  300  :  58  instead  of  57. 

553  :  113  : :  300  :  61 

545  :  112  :  :  300  :  61 

493  :  110  :  :  300  :  67 
We  find,  therefore,  that  in  the  case  of  both  these  systems  of  metals,  the  tension  slowly 
rises  with  increase  of  temperature,  being  much  better  marked  in  the  latter  than  in  the 
former  instance. 


96 


QUANTITY  OF  ELECTRICITY  INDEPENDENT  OF  HEATED  SURFACE. 


361.  The  increase  of  tension  here  detected  depends  unquestionably  on  increased  re- 
sistance to  conduction,  which  the  wires  exhibit  as  their  temperature  rises,  as  the  follow- 
ing experiments  show. 

362.  A  pair  of  copper  and  iron  evolved  a  current  at  the  boiling  point  of  water,  which, 
passing  through  a  wire  of  copper  eight  feet  long,  was  determined  at  the  galvanometer  to 
be  176  degrees.  Having  twisted  a  part  of  this  wire  into  a  spiral,  so  as  to  go  over  the 
flame  of  a  spirit-lamp,  8  inches  of  it  were  thereby  brought  to  a  red  heat ;  the  deviation 
of  the  needle  fell  now  to  165,  being  a  deficit  of  11  degrees.  In  this  experiment,  care  was 
taken  that  no  heat  should  be  transmitted  along  the  wire  to  the  connecting  cups. 

363.  The  same  was  repeated  with  a  piece  of  iron  wire,  of  the  same  length  and  un- 
der the  same  circumstances.  The  current  at  first  being  90  degrees,  as  soon  as  the  spiral 
was  made  red  hot,  it  fell  to  61  degrees,  being  a  deficit,  therefore,  of  nearly  one  third  the 
whole  amount. 

364.  To  the  increased  resistance  to  conduction,  occasioned  by  an  increased  tempera- 
ture, we  are  to  impute  the  slight  rise  of  tension  observed  in  thermo-electric  currents. 
The  quantities  are  of  the  same  order. 

365.  We  have  next  to  show  "  that  the  quantity  of  electricity  evolved  at  any  given 
temperature  is  independent  of  the  amount  of  heated  surface ;  a  mere  point  being  just 
as  efficacious  as  an  indefinitely  extended  surface." 

366.  The  quantities  of  electricity  evolved  by  hydro-electric  pairs  has  been  shown  to 
increase  with  their  surfaces,  but  it  is  not  so  in  thermo-electric  arrangements.  A  pair 
of  disks  of  copper  and  iron  two  inches  in  diameter  were  soldered  together ;  they  had 
continuous  straps  projecting  from  them,  which  served  to  connect  them  with  the  galva- 
nometer cups.  At  the  boiling  point  of  water  they  gave  62  degrees  ;  on  being  cut  down 
to  half  an  inch  in  diameter,  they  still  gave  62.  On  the  disk  being  entirely  removed, 
and  the  copper  made  to  touch  the  iron  by  a  mere  point,  its  extremity  being  roughly 
sharpened,  the  deflection  was  still  62. 

367.  By  means  of  a  common  deflecting  multiplier,  I  obtained  the  following  results : 
1st.  A  copper  wire  being  placed  in  a  bath  of  mercury,  the  temperature  of  which  was 
240°  Fah.,  I  dipped  into  it  a  second  copper  wire,  the  temperature  of  which  was  about 
60"  Fah.;  the  galvanometer  needles  moved  through  15  degrees. 

2d.  The  cold  wire  being  sharpened  to  a  point,  and  plunged  deliberately  into  the 
mercury  to  the  bottom  of  the  bath,  the  deflection  was  19  degrees. 

3d.  But  when  I  touched  the  surface  of  the  mercury  with  the  very  iioint  of  the  cold 
wire,  there  was  a  deflection  of  60  degrees. 

368.  Having  laid  a  plate  of  tinned  iron  upon  the  surface  of  some  hot  mercury,  it 
was  touched  with  the  point  of  the  cold  wire.  There  was  a  strong  deflection  of  the 
needles  in  the  opposite  direction  to  what  would  have  been  the  case  had  the  mercury 
been  touched,  and  not  the  iron.  The  under  surface  of  the  iron  was  therefore  acting  as 
a  hot  face,  and  the  parts  round  the  point  as  a  cold  face,  being  temporarily  chilled  by  the 
touch  of  the  wire. 

369.  These  results  explain  the  anomalies  observed  by  some  of  those  who  investi- 
gated the  course  of  thermo-electric  currents  by  means  of  small  metallic  fragments. 


QUANTITY  OF  ELECTRICITY  PROPORTIONAL  TO  NUMBER  OF  PAIRS. 


97 


370.  It  would  therefore  seem  that,  when  wires  of  the  same  metal  are  used  as  electro- 
motors, the  quantity  of  electricity  evolved  depends  on  the  quantity  of  caloric  that  can  be 
communicated  in  a  given  time.  Time,  therefore,  under  these  circumstances,  tnust  enter 
as  an  element  of  thermo-electric  action.  In  the  case  of  a  single  metal,  the  maximum 
effect  would  be  produced  when  the  hot  element  is  a  mass,  and  the  cold  one  a  point. 

371.  And,  lastly,  "  That  the  quantities  of  electricity  evolved  in  a  pile  of  pairs  are 
directly  proportional  to  the  number  of  the  elements." 

372.  In  the  first  trials  which  I  made  to  determine  the  effect  of  increasing  the  num- 
ber of  pairs  in  a  pile,  the  results  obtained  were  contradictory ;  by  operating,  however, 
in  the  following  way,  the  proposition  was  at  last  satisfactorily  determined  : 

1st.  The  resistance  to  conduction  was  made  nearly  constant  by  uniting  all  the  pairs 
intended  to  be  worked  with  at  once.  The  current,  therefore,  whether  generated  by 
one,  two,  three,  four,  or  more  pairs,  had  always  to  run  through  the  same  length  of  wire, 
and  experienced  in  all  cases  a  uniform  resistance. 

2dly.  By  making  each  individual  element  of  considerable  length,  the  liabiHty  of  trans- 
mission from  the  hot  to  the  cold  extremity  was  diminished. 

373.  Having,  therefore,  taken  six  pairs  of  copper  and  iron  wires,  one  sixteenth  of  an 
inch  thick,  and  each  element  38  inches  long,  I  formed  them,  by  soldering  their  alter- 
nate ends,  into  a  continuous  battery.  Then  I  successively  immersed  in  boiling  water 
one,  two,  three,  &c.,  of  the  extremities,  their  length  allowing  freedom  of  motion,  and 
the  other  extremities  not  differing  perceptibly  from  the  temperature  of  the  room. 

374.  The  following  table  exhibits  one  of  this  series  of  experiments  : 


TABLE  IV. 


No.  of  Pairs. 

Calculated  Deviations. 

Observed  Deviations. 

1. 

55 

55 

2. 

110 

111 

3. 

165 

165 

4. 

220 

220 

5. 

275 

272 

6. 

330 

332 

Hence  there  cannot  be  any  doubt  that  the  quantities  of  electricity  evolved  by  com- 
pound batteries,  at  the  same  temperature,  are  directly  proportional  to  the  number  of  the 
pairs. 

375.  With  some  general  remarks,  arising  from  the  foregoing  subjects,  I  shall  conclude 
this  chapter, 

376.  It  is  of  importance  to  remember  that  thermo-electric  currents  traverse  metallic 
masses  only  on  account  of  differences  of  temperature  existing  at  different  points. 

377.  When  a  current  of  electricity,  flowing  from  the  poles  of  a  battery,  is  made  to 
traverse  a  metallic  sheet,  the  whole  of  it  does  not  pass  in  a  straight  line  from  one  pole 
to  the  other,  but  diffuses  itself  through  the  metal,  diverging  from  one  point  and  conver- 
ging to  the  other.  The  greater  part  of  the  current  is  found,  however,  to  take  the  shortest 
route. 

378.  Combining,  therefore,  the  foregoing  observations,  we  perceive  that  there  are 
certain  forms  of  construction  which  will  give  to  thermo-electric  arrangements  peculiar 
advantages.   For  example,  the  surfaces  united  by  soldering  must  not  be  too  massive.  Let 

N 


98'  BEST  FORMS  OF  CONSTRUCTING  THERMOMETRIC  PAIRS. 

a,  jig.  35,  oe  a  bar  of  antimony,  and  h  a  bar  of  bismuth  ;  let  them  be  soldered  together 
along  the  line  c  d,  and  at  the  point  d  let  the  temperature  be  raised ;  a  current  is  imme- 
diately excited  ;  but  this  does  not  pass  round  the  bars  a,  h,  inasmuch  as  it  finds  a  shorter 
and  readier  channel  through  the  metals,  between  c  and  d,  circulating,  therefore,  as  in- 
dicated by  the  arrows.  Nor  will  the  whole  current  pass  round  the  bars  until  the  tem- 
perature of  the  soldered  surface  has  become  uniform. 

379.  An  obvious  improvement  in  such  a  combination  is  shown  in  Jig.  36,  which 
consists  of  the  former  arrangement  cut  out  along  the  dotted  lines  :  here  the  whole  cur- 
rent, so  soon  as  it  exists,  is  forced  to  pass  along  the  bars  ;  and  because  the  mass  of 
metal  has  been  diminished  at  the  line  of  junction,  such  a  pair  will  change  its  tempera- 
ture very  quickly. 

380.  One  of  the  very  best  forms  for  a  thermo-electric  couple  is  given  in  Jig.  37,  where 
a  is  a  semi-cylindrical  bar  of  antimony,  h  one  of  bismuth,  united  together  by  the  opposite 
corners  of  a  lozenge-shaped  piece  of  copper,  c.  From  its  exposing  so  much  surface, 
the  copper  becomes  hot  and  cold  with  the  greatest  promptitude,  and,  from  its  good  con- 
ducting power,  it  may  be  made  very  thin  without  injury  to  the  current.  With  a  pair 
of  bars  three  fourths  of  an  inch  thick,  and  a  circular  copper  plate,  c,  having  both  sur- 
faces blackened,  I  have  repeated  the  greater  part  of  those  experiments  which  M.  Mel- 
LONi  made  with  his  multiplier. 

381.  The  currents  which  circulate  in  a  steel  magnet  are  to  all  appearance  perpetual. 
I  thought  for  some  time  it  might  be  possible  to  procure  similar  perpetual  currents  by 
compound  thermo-electric  arrangements.  Fig.  38  will  serve  to  show  the  character  of 
these  combinations,  and  also  the  cause  of  their  failure.  Let  a,  b,  c,  be  wires  of  three 
different  metals,  soldered  together  so  as  to  form  a  triangle.  Now,  if  these  metals  were 
selected,  so  that  a  and  b  could  form  a  more  powerful  thermo-electric  pair  than  a  and  c 
or  b  and  c,  it  might  be  expected  that  at  all  temperatures  an  incessant  current  would  run 
round  the  system.  Such,  however,  will  not  be  found  to  be  the  case.  In  effect,  any 
one  of  these  three  serves  simply  as  a  connecting  solder  to  the  other  two,  and  hence  no 
current  is  excited ;  for  the  ends  that  have  the  third  metal  between  them,  although  that 
metal  intervenes,  are  under  exactly  the  same  condition  as  the  other  ends  which  are  in 
contact. 

382.  Thermo-electric  currents,  evolved  by  pairs  of  different  metals,  do  not  appear  to 
differ  specifically.  As  different  gases  during  combustion  burn  with  differently-coloured 
flames,  and  as  different  sources  of  caloric  evolve  rays  of  heat  which  are  absorbed  dif- 
ferently by  different  media,  it  might  be  expected  that  a  pair  of  wires  of  copper  and  pla- 
tina  would  give  out  a  current  of  electricity  unlike  that  of  iron  and  palladium.  I  have 
made  many  trials  on  this  point,  adjusting  a  wire  of  copper  and  one  of  lead  to  each  other, 
so  as  to  stop  equal  quantities  of  electricity  flowing  from  a  pair  of  copper  and  platina, 
the  galvanometer  needles  being  brought  to  the  same  point,  whether  the  long  wire  of 
copper  or  the  short  wire  of  lead  was  employed.  But,  in  the  case  of  every  combination 
which  I  tried,  these  two  wires  acted  alike,  nor  could  I  ever  evolve  a  current  which 
would  pass  with  more  or  less  resistance  along  the  lead  than  along  the  copper. 


ACTION  OF  ABSORBENT  MEDIA  AND  IDEAL  COLOURATION. 


99 


CHAPTER  X. 

EXPERIMENTS   ON   THE    CHEMICAL   ACTION   OF   SOLAR  LIGHT. 

(From  the  Journal  of  the  Franklin  Institute  for  June,  July,  August,  and  September,  1837.) 

Contents:  Action  of  Absorbent  Media. — Ideal  Colouration  of  the  Chemical  Rays. — 
Specific  Absorption. — Colorific  Absorption. — Calorific  Absorption. — Specific  Absorp- 
tion of  the  Chemical  Rays. — Effect  of  Yellow  Solutions. 

Decomposition  of  Carbonic  Acid  by  Leaves.  —  Penetration  of  Dimensions  in  Gases.  — 
Decomposition  of  Carbonic  Acid  under  various  coloured  Media. — Gas  from  Leaves 
contains  Nitrogen. — Chemical  Rays  of  different  Colours. — Identity  of  Volume  in  the 
absorbed  and  evolved  Gas. — Cause  of  the  Decomposition. 

Hitter  s  Experiments  of  the  Non-oxygenation  of  Phosphorus. 

Decomposition  of  the  Salts  of  Silver. — Prismatic  Spectrum  on  Bro7nide  of  Silver. — In- 
terference of  Chemical  Rays. — Salts  decomposed  by  Light. — Moonlight  and  Artificial 
Flames  are  Inactive. 

Of  Perihelion  Motions. — Deiv  of  Water  and  Mercury. — Iodine. — Chloride  of  Gold. — 
Non-deposition  on  a  Glass  Plate. — Current  Action. — Actio7i  of  Flame. — Action  of 
Metal  Screens. — Protecting  Action  of  a  Metal  Ring. — Is  there  Electricity  in  the 
Solar  Ray? 

Cause  of  the  Green  Colour  of  Leaves. —  Plants  groio  in  Lights  of  various  Colours. — 
Seeds  also  germinate  in  Red,  Yellow,  and  Blue  Light. — Chemical  Rays  of  different 
Colours. 

383.  The  efifect  of  absorbent  media  upon  the  colorific  rays  of  light  has  been,  as  was 
predicted  by  an  eminent  writer  on  optics,  of  singular  service  in  developing  new  views 
of  this  subtle  agent,  and  giving  us  a  more  precise  knowledge  of  the  complex  constitu- 
tion of  the  solar  beam.  Hitherto,  the  action  of  these  media  upon  the  calorific  and 
chemical  rays  has  not  been  thoroughly  investigated,  nor  are  there,  so  far  as  I  know, 
any  experiments  on  record  exhibiting  this  matter  in  its  full  importance. 

384.  We  have  been  accustomed  to  regard  the  chemical  properties  of  the  solar  spec- 
trum as  due  to  the  violet  ray,  A  similar  opinion  was  formerly  maintained  respecting 
the  calorific  constitution  of  the  red  ray.  The  position  to  which  we  are  brought  by 
advanced  investigation  has  long  ago  established  the  separate  existence  of  heat-making 
rays,  and  the  experiments  here  communicated  give  much  weight  to  the  doctrine  that 
the  chemical  rays  have  also  a  separate  existence.  It  is  true  it  cannot  yet  be  proved, 
though  analogy  and  probability  are  favourable  to  the  idea  that  there  are  subdivisions 
both  of  the  chemical  and  calorific  rays,  similar  to  those  of  which  our  senses  give  evi- 
dence in  the  colorific  ray,  each  of  which  is  endued  with  distinct  powers  of  its  own. 

385.  How  complex  is,  then,  the  constitution  of  the  solar  beam  !  a  ray  of  heat,  com- 
posed, perhaps,  of  three  or  more  rays  of  different  refrangibility ;  a  ray  of  light,  composed 


100 


SPECIFIC  ABSORPTION.— INSTRUMENTAL  ARRANGEMENTS. 


of  three  simpler  rajs ;  a  ray  endued  with  chemical  energy,  and  of  a  similar  composi- 
tion to  the  former,  as  analogy  would  lead  us  to  suspect.  Again,  each  of  these  element- 
ary rays  is  composed  of  particles,  one  half  of  which  have  their  planes  of  polarization 
at  right  angles  to  the  other.  All  these  elements  taken  together  constitute  a  heam 
of  the  sun's  light.  Emanations  from  the  sun,  after  they  have  undergone  the  absorptive 
action  of  the  atmosphere  of  that  great  luminary,  and  of  that  of  the  earth,  still  reach  us  in 
abundance,  accompanying  his  light,  and  traversing  the  great  vacuum,  perhaps  as  far  as 
his  attraction  is  felt. 

386.  If  we  take  a  coloured  medium  of  any  kind,  and  transmit  through  it  a  beam  of 
the  sun's  light,  we  find,  on  examination,  that  certain  of  the  rays  exciting  vision  are 
absorbed,  that  the  light  which  passes  through  is  not  homogeneous,  for  it  is  capable 
of  decomposition  by  the  prism ;  it  is  a  compound  coloured  ray,  consisting  of  all  the 
rays,  complementary  to  those  which  the  medium  has  absorbed.  Nor  is  this  absorbing 
effect  confined  to  the  rays  producing  vision;  the  rays  of  heat  suffer  in  like  manner;  some- 
times those  which  are  more  refrangible  are  wanting,  sometimes  those  which  are  of  less, 
or  of  medium  refrangibility,  are  absent.  Often,  at  the  same  time,  do  the  chemical  rays 
sustain  a  similar  attack.  There  are  solutions  and  media  transparent  to  light,  and 
nearly  opaque  to  heat ;  there  are  others  transparent  to  light  and  to  heat,  and  opaque  to 
the  chemical  ray.  It  is  from  these  facts  that  we  are  able  to  establish  the  separate 
existence  of  three  genera  of  rays  in  the  sunbeam,  each  of  which  is  essentially  distinct 
in  its  properties,  and  different  in  its  mode  of  action  from  the  others.  Our  eye  can  de- 
tect, in  the  rays  exciting  vision,  difference  of  constitution,  because  we  are  able  to  per- 
ceive a  difference  of  colour.  Had  we  specific  organs  for  indicating  difference  in  the 
heat-making,  or  chemical  rays,  perhaps  we  might  find  in  them  a  similar  constitution. 

387.  It  is  between  three  and  four  years  since  that  the  investigation  which  forms  the 
subject  of  this  memoir  was  first  commenced,  under  the  form  of  an  examination  of  the 
properties  of  the  chemical  ray.  In  the  Journal  of  the  Franklin  Institute  some  of  the 
earlier  results  are  recorded,  and  among  them  the  fact,  that  the  crystallization  of  cam- 
phor, which  has  long  been  known  to  take  place  on  the  enlightened  sides  of  vessels 
exposed  to  the  sun,  occurs  with  very  great  rapidity,  if  the  glass  in  which  it  is  tried  be 
exhausted  of  air.  In  tracing  out  this  fact,  to  ascertain  its  cause,  a  field  of  no  common 
interest  has  been  entered.  I  do  not  here  present  a  record  of  the  facts  as  they  were 
successively  developed  by  an  analysis  of  the  phenomena,  but  place  them  in  that  order 
which  appears  to  me  the  best  to  obtain  a  true  estimate  of  their  bearing. 

388.  Into  a  darkened  chamber,  the  shutter  of  which  is  seen  in  section  at  a  a,  fig.  39, 
a  beam  of  the  sun's  light  may  be  made  to  pass  horizontally,  by  means  of  a  mirror 
of  silvered  glass,  c.  The  mirror  which  I  use  is  one  belonging  to  a  solar  microscope, 
and  by  turning  the  milled  screws,  e  e,  it  can  be  brought  into  any  position  required  to 
throw  a  beam  horizontally  into  the  room,  no  matter  what  may  be  the  place  of  the  sun. 
A  brass  tube,/,  belonging  to  the  same  instrument,  and  two  inches  in  diameter,  can 
be  screwed  into  the  position  figured,  if  desirable  ;  there  is  also  a  lens,  g,  which  may  oc- 
casionally l?e  fixed  at  g ;  its  focus  is  nine  inches,  its  diameter  about  two  inches,  and  the 
diameter  of  the  sun's  image  rV  of  an  inch. 


COLORIFIC  ABSORPTION.  101 

389.  A  piece  of  sheet  lead,  about  a  quarter  of  an  inch  thick,  is  to  be  cut  into  the 
form  of  a  horse-shoe,  of  such  magnitude  that  a  circle  one  inch  in  diameter  might  be 
inscribed  in  it.  Upon  this  lead  two  pieces  of  very  pure  and  transparent  crown  glass 
are  cemented,  so  as  to  form  a  trough,  for  containing  a  variety  of  liquids.  It  is  well  to 
accommodate  this  trough  with  a  strong  foot,  or  basis,  a  a,  and  several  such  troughs  may 
be  provided.    Fig.  40,  c  c  c,  the  leaden  horse-shoe  ;  h  b,  the  glass  plates. 

390.  A  thin  metallic  plate,  three  or  four  inches  square,  having  a  longitudinal  slit 
about  one  inch  long  and  tV  inch  wide  in  it,  is  also  to  be  provided.  It  is  convenient 
that  this,  too,  should  be  furnished  with  a  pediment.    Fig.  4:1,  a  a,  the  slit. 

391.  The  lens  (o-,  Jig.  39^  having  been  removed,  by  turning  the  screws  a  beam  of 
light  is  to  be  thrown  horizontally  into  the  room;  the  screen  {Jig.  41)  is  then  to  be  placed 
before  the  brass  tube,  so  that  the  slit  in  it  may  allow  a  narrow  streak  of  light  to  pass. 
The  trough  {_fig.  40)  is  then  placed  behind,  in  such  a  position  that  half  the  light  which 
comes  through  the  slit  in  the  screen  may  pass  through  the  liquid  contained  in  the 
trough,  and  the  other  half  pass  by  its  side  unintercepted.  This  arrangement  is  shown 
in^^.  42.  Behind  the  trough  is  placed  a  flint  glass  prism,  d,Jig.  43,  and,  farther  still, 
a  white  pasteboard  screen,  e,  of  suitable  dimensions,  a  being  the  metallic  screen,  h  the 
trough. 

392.  The  action  of  this  arrangement  is  as  follows  :  the  beam  of  light  cast  by  the 
mirror  into  the  room  is  entirely  intercepted,  except  the  small  portion  which  passed 
through  the  slit  in  the  metallic  screen.  A  part  of  this  passes  through  the  trough,  and 
a  part  on  one  side  of  it,  the  middle  part  being  obstructed  by  the  leaden  horseshoe. 
Two  beams  of  light,  therefore,  fall  on  the  prism,  one  of  which  has  passed  through  the 
trough,  and  one  which  has  not,  and  they  are  separated  from  each  other  by  a  dark  in- 
terval. The  prism  disperses  both,  and  there  fall  on  the  pasteboard  screen  two  spectra, 
side  by  side,  close  enough  for  a  very  accurate  examination.  One  of  them  has  been 
acted  on  by  the  fluid  in  the  trough,  the  other  is  undisturbed.  In  my  arrangement,  the 
spectrum  a  happens  to  be  the  natural  one,  and  h  the  disturbed  one  {_fig.  44). 

393.  Let  us  now  take  an  example,  as  an  illustration  of  the  use  of  this  apparatus. 
Fill  the  trough  with  distilled  water,  and  let  the  mirror  throw  a  horizontal  beam. 
Two  spectra  are  seen  on  the  screen,  e,Jig.  43,  close  to  each  other,  side  by  side,  with  a 
dark  interval  between  them.  They  contain,  as  may  be  perceived,  all  the  seven  col- 
ours of  Newton,  nor  does  the  one  difler  in  any  wise  from  the  other,  as  in^jg-.  45. 

394.  Having  poured  the  water  out  of  the  trough,  fill  it  with  a  strong,  but  clear  solu- 
tion of  the  chromate  of  potassa  ;  on  looking  at  the  spectra  on  the  screen, «  is  still  found 
of  its  natural  appearance,  but  by  the  side  of  it  there  is  a  distorted  spectrum,  formed  by 
the  light  that  has  passed  through  the  trough  ;  the  blue,  the  indigo,  and  the  violet  rays 
are  wanting,  as  is  seen  in^jg".  46.  These  colours  have  then  been  absorbed  by  the  so- 
lution of  chromate  of  potassa.  If  this  solution  be  poured  out,  and  one  of  sulphate  of 
copper  and  ammonia  poured  into  the  trough,  another  kind  of  spectrum  is  produced, 
where  the  red  and  much  of  the  yellow  light  is  wanting,  as  '\n  jig.  47.  If  a  strong  solu- 
tion of  Brazil  wood  is  used,  the  disturbed  spectrum  will  be  found  to  have  lost  its  violet, 
indigo,  blue,  green,  yellow,  and  a  great  part  of  its  orange  rays,  as  represented  in  Jig.  48. 


102 


COLORIFIC  ABSORPTION. 


395.  By  having  the  two  spectra  side  by  side,  and  close  to  one  another,  they  are 
placed  under  circumstances  most  convenient  for  making  a  perfect  comparative  estimate 
of  the  light  which  is  lost.  In  this  manner  the  following  table  has  been  constructed. 
The  specific  gravity  of  the  solutions  is  not  given,  as  it  is  not  supposed  that  any  direct 
connexion  exists  between  the  density  of  a  solution  and  its  absorptive  power.  Much 
more  depends  on  the  shade  of  colour. 


TABLE  OF  COLORIFIC  RAYS  ABSORBED  BY  SOLUTIONS. 


Name. 

Rays  absorbed. 

Bichromate  of  potassa  

blue,  violet. 

Prussiate  of  potassa  

extreme  red,  extreme  violet,  yellow. 

extreme  red. 

violet. 

extreme  violet. 

Sulp.  copper  and  ammonia    .    .    .  . 

red,  yellow. 

violet,  indigo  blue,  orange,  and  a  part  of  green. 

orange,  yellow,  green,  extreme  violet. 

Chromate  of  potassa  

extreme  red,  blue,  violet. 

violet,  indigo  blue. 

Hydro-sulphate  of  lime  

violet,  blue. 

Decoc.  logwood  in  alum-water  . 

orange,  yellow,  blue,  and  green. 

Decoc.  of  Brazil  wood  

violet,  indigo  blue,  green,  yellow,  orange. 

Cochineal  in  cream  of  tartar  solution  . 

yellow  and  part  blue. 

396.  Some  remarkable  phenomena  may  be  produced  by  taking  double  solutions  ;  a 
beam  which  has  passed  through  a  stratum  of  solution  of  sulphate  of  copper  and  ammo- 
nia, and  then  through  a  decoction  of  Brazil  wood,  becomes  almost  totally  extinct. 
On  looking  through  such  solutions  separately  at  the  noontide  sun,  he  appears  with 
overpowering  effulgence,  but  on  using  them  together,  only  a  very  faint  trace  of  a  dirty 
olive-green  light  indicates  his  position.  The  sulphate  of  copper  and  ammonia  absorbs 
the  red  rays,  and  the  Brazil-wood  decoction  nearly  all  the  remainder. 

397.  Already  have  some  of  these  phenomena  of  absorption,  in  the  hands  of  Sir  D. 
Brewster,  disclosed  important  facts  respecting  the  colorific  rays.  The  colour  of  the 
sky,  and  of  the  clouds,  and  of  the  sea,  has  also  been  long  attributed  to  an  action  of  this 
kind,  exercised  by  thick  masses  of  air,  or  vapour,  or  water. 

398.  But  this  action  is  not  alone  confined  to  the  rays  producing  vision,  it  extends  to 
the  other  elementary  constituents  of  the  spectrum.  While  the  trough  (h,Jig.  43)  is  filled 
with  a  solution  of  sulphate  of  copper  and  ammonia,  if  the  prism  and  the  metallic  screen 
be  removed,  and  a  very  delicate  thermometer  be  plunged  in  the  ray,  a  new  phenome- 
non is  discovered ;  the  ray  is  found  to  be,  to  a  great  extent,  deprived  of  the  power  of 
exciting  heat,  and  the  thermometer  shows  little  disposition  to  rise.  How  is  this  !  is  it 
because  the  red  ray  is  gone  that  the  sunbeam  has  lost  its  power  of  exciting  a  sensa- 
tion of  warmth  1  It  was  at  one  time  supposed  that  as  the  violet  ray  had  the  power  of 
determining  chemical  change,  so  the  red  ray  possessed  the  power  of  exciting  calorific 
impressions. 

399.  Fill  the  trough  next  with  a  strong  decoction  of  Brazil  wood,  analyze  the  light 
which  passes  through  it  by  the  prism  (391),  and  it  will  be  found  that  all  the  rays  have 
been  absorbed  except  the  red.  Now,  in  such  a  beam,  if  the  red  ray  possess  inherent 
caloric,  the  thermometer  should  rise  as  much,  or  nearly  as  much,  as  if  it  were  in  the 
direct  solar  ray  ;  if  the  colour  passes  unabsorbed,  so  too  should  the  caloric  ;  but,  place 
the  thermometer  in  the  beam,  and  it  does  not  rise.    Or  throw  a  concentrated  column 


CALORIFIC  ABSORPTION.  j^Q3 

of  such  light  upon  it  hy  a  convex  lens,  and  it  is  still  unmoved.  We  are  therefore  for- 
ced to  conclude  that  the  rajs  exciting  heat  are  independent  of  those  exciting  vision  ; 
that  neither  the  red,  nor  the  yellow,  nor  the  hlue  possesses  inherent  caloric  ;  and,  more- 
over, that  substances  may  he  transparent  to  red,  to  yellow,  or  to  blue  light,  or  to  all,  mikI 
yet  more  or  less  opaque  to  the  rays  of  heat. 

400.  It  is  not  alone  among  watery  solutions  or  alcoholic  tinctures  that  we  find 
abundant  instances  of  this  kind  of  action;  the  mineral  kingdom  furnishes  many.  A 
very  thin  lamina  of  pitch  is  transparent  to  red  light,  but  almost  opaque  to  the  rays  of 
heat.  I  have  examined  a  variety  of  bodies,  gaseous,  liquid,  and  solid,  and  shall  here 
point  out  the  method  which  has  been  followed  in  obtaining  the  results  contained  in  the 
following  table. 

401.  The  mirror  being  placed  upon  the  shutter,  as  in  (388),  a  plano-convex  lens  is 
to  be  screwed  into  the  tube,  so  as  to  bring  the  rays  to  a  focus  on  one  of  the  balls  of  a 
very  delicate  differential  thermometer ;  the  motion  of  the  fluid  is  rapid,  and  the  instru- 
ment soon  attains  a  position  of  equilibrium :  this  gives  the  heat  of  the  sunbeam  as  con- 
centrated by  the  lens.  To  find  the  effect  of  any  liquid  medium  in  absorbing  these  rays, 
the  trough  filled  with  the  substance  under  trial  is  placed  at  the  extremity  of  the  brass 
tube,  in  a  position  as  at  c  {fig-  50).  The  cone  of  rays  converges  from  the  lens,  a,  on 
the  ball  b ;  but  because  the  trough  has  plain  and  parallel  surfaces,  the  rays  still  pass  on, 
and  form  an  image  on  the  focal  ball  of  the  thermometer.  The  total  effect,  as  given  by 
the  expansion  of  the  air  in  the  instrument,  and  which  has  formed  the  basis  of  the  fol- 
lowing table,  is  not,  however,  an  exact  estimate  of  the  action  of  the  liquid  solution.  In 
the  instrument  which  I  am  in  the  habit  of  using,  the  convex  lens  is  of  flint  glass,  and 
the  plates  of  the  trough  of  Boston  crown  glass ;  there  are,  therefore,  at  least  two  disturb- 
ances, the  absorbing  action  of  the  former,  and  still  more  powerful  effect  of  the  latter. 
It  has  been  considered,  from  the  experiments  of  Melloni,  that  the  power  of  absorption 
was  inversely  as  the  power  of  refraction,  but  whether  an  extended  train  of  investiga- 
tions will  corroborate  this  supposition,  remains  to  be  seen.  In  the  following  experi- 
ments, the  instrumental  arrangement  being  always  identical,  a  comparison  may  be  insti- 
tuted of  the  action  of  any  two  of  the  solutions  ;  but  the  absolute  action  of  each  cannot 
be  determined,  except  after  allowing  for  the  additional  effect  of  the  flint  glass  lens  and 
the  crown  glass  plates.  In  practice,  it  will  be  found  very  useful  to  blacken  the  focal 
ball  of  the  thermometer,  as  seen  at  b  {_fig.  50).  It  serves  to  give  a  larger  scale  of  ther- 
mometric  expansion.  It  is  also  requisite  to  cover  the  thermometer  with  a  very  thin  case 
of  pure  and  transparent  glass,  which  prevents  the  disturbance  of  currents,  and  also  of 
the  heat  radiated  from  other  bodies  in  the  vicinity ;  this  introduces,  however,  the  ab- 
sorptive action  of  a  third  plate  of  glass ;  b  d'ls  the  thermometer,  and  e  e  the  glass  cover 
{fig.b^). 

402.  By  these  arrangements,  it  was  found  that  a  thin  stratum  of  pitch  enclosed  be- 
tween two  plates  of  crown  glass,  and  which  transmitted  a  homogeneous  red  light,  ab- 
sorbing all  the  other  colours  of  the  spectrum,  allowed  only  nineteen  rays  of  heat  to  pass 
through  of  every  hundred  that  fell  upon  it. 

403.  A  solution  of  the  sulphate  of  copper  and  ammonia,  which  absorbs  the  red  and 


104 


ABSORPTION  OF  HEAT  AND  CHEMICAL  RAYS. 


the  yellow  light,  being  operated  upon  in  like  manner,  was  found  to  transmit  twenty 
rays  out  of  every  hundred  that  fell  upon  it. 

404.  There  is,  however,  considerable  difficulty  in  obtaining  these  numerical  results 
with  accuracy,  arising  partly  from  the  difficulty  of  obtaining  specimens  of  exactly  the 
same  composition,  but  more  especially  owing  to  changes  taking  place  in  their  colour. 
In  process  of  time,  most  vegetable  solutions  undergo  spontaneous  changes,  and  no  lon- 
ger give  the  same  results.  But,  where  the  same  sample  is  operated  on,  under  the  same 
circumstances,  repeated  experiment  assures  me  that  this  arrangement  gives  comparable 
indications. 

405.  Vapours  and  gases  may  also  be  put  under  trial.  The  vapour  of  iodine,  the 
spectrum  of  which  is  remarkable  as  containing  only  the  extreme  rays,  and  wanting  those 
of  medium  refraugibihty  {fig.  49),  absorbs  two  thirds  of  the  heat  that  impinges  on  it. 
The  vapour  of  nitrous  acid,  which  stops  the  violet,  bhie,  indigo,  and  yellow  light  {fig. 
51),  has  a  similar  effect  on  the  heat.  To  experiment  upon  these  bodies,  a  cubical  bot- 
tle {fig.  52)  is  very  convenient  to  generate  the  vapour  in,  and  also  to  transmit  the 
light  through  ;  it  will  then  replace  the  trough  of  (389).  Nitrous  acid  vapour  is  best 
made  for  these  purposes  from  nitrate  of  lead. 

406.  Having  prepared  a  variety  of  solutions  for  the  purpose  of  experiment,  and  using 
for  each  the  same  trough,  thoroughly  cleansed  after  each  trial,  the  following  table  will 
give  an  estimate  of  the  results  obtained;  it  is  arranged  according  to  the  power  of  each 
solution,  the  first  on  the  list  being  the  most  energetic : 


TABLE  OF  THE  THERMO-ABSORPTIVE  POWER  OF  SOLUTIONS. 


Decoction  of  logwood  in  alum  water, 

Bichromate  of  potassa. 

Solution  of  sulphate  of  copper  and  ammonia, 

Hydro-sulphate  of  lime, 

Iiitmus  water. 

Muriate  of  iron. 

Decoction  of  Brazil  wood, 

on  of  turpentine. 

Decoction  of  cochineal. 

Prussiate  of  potassa. 

Solution  of  tannin, 

Sulphate  of  copper. 

Solution  of  chloride  of  chromium, 

Chloride  of  platinum, 

Tincture  of  turmeric, 

Chloride  of  gold, 

Tincture  of  saffron. 

Oil  of  bergamot, 

Sulpho-cyanate  of  iron, 

Linseed  oil. 

Hydro-sulphate  of  ammonia, 

Nitrous  ether, 

Muriate  of  cohalt, 

Water. 

407.  Still  more  powerful  effects  are  produced  by  making  binary  or  ternary  arrange- 
ments. If,  for  instance,  a  beam  of  the  sun  falls  upon  a  very  thin,  transparent  stratum  of 
pitch,  and  then  passes  through  a  solution  of  sulphate  of  copper  and  ammonia,  or  through 
linseed  oil,  not  more  than  one  fortieth  part  of  the  caloric  is  transmitted. 

408.  A  question  here  naturally  arises :  What  becomes  of  the  heat  thus  lost  ?  does 
it  enter  into  such  a  combination  with  these  media  as  to  be  detected  in  them  by  the 
thermometer  as  sensible  heat  1  This  is  a  question  of  much  difficulty ;  there  are  so 
many  disturbing  causes  in  operation,  the  general  results  of  experiment  have  not  yet 
furnished  actual  proof  that  the  heat  missing  is  to  be  foimd  in  the  fluid  solutions.  I 
would  not,  however,  be  understood  to  deny  that  such  is  the  case ;  only  that,  at  present, 
our  information  does  not  warrant  such  a  conclusion.  It  might  be  supposed  that  these 
solutions  do  not  act  by  a  proper  absorptive  power,  but  merely  offer  that  kind  of  obstacle 
to  the  transmission  of  heat  that  turbid  media  do  to  light.    Not  only,  however,  does 


I 


EFFECTS  OF  YELLOW  SOLUTIONS  ON  THE  CHEMICAL  RAYS. 


105 


direct  experiment  discountenance  this,  but  the  analogy  of  their  action  on  the  chemical 
ray  renders  it  extremely  improbable,  an  action  which  I  proceed  to  develop,  ^g-.  50, 
being  still  consulted. 

409.  Having  removed  the  differential  thermometer  and  its  case,  and  produced  a  cone 
of  light  converging  from  the  lens,  and  passed  it  through  a  solution  of  sulphate  of  copper 
and  ammonia  contained  in  the  trough,  if  now  we  hold  in  the  focus  a  piece  of  bibu- 
lous paper  imbued  with  chloride  of  silver,  although  little  or  no  heat  is  transmitted 
through  the  solution,  yet  an  extremely  dark  spot  is  produced,  characteristic  of  the  black- 
ening of  that  substance  by  the  solar  rays.  Though,  therefore,  the  double  salt  transuiits 
the  ray  of  heat  with  difficulty,  the  ray  of  chemical  action  passes  with  great  facility. 
If  a  trough  containing  a  strong  solution  of  bichromate  of  potassa  be  now  substituted,  a 
far  larger  quantity  of  light  will  pass,  and  vastly  more  heat ;  but  a  paper  imbued  with 
chloride  of  silver  being  laid  in  the  locus,  no  chemical  change  lohatever  goes  on,  the 
chloride  retaining  its  usual  whiteness. 

410.  I  placed  a  piece  of  paper  imbued  with  chloride  of  silver  in  a  cubical  box,  one 
of  the  sides  of  which  was  formed  of  a  pair  of  glass  plates,  with  a  solution  of  bichromate 
of  potassa  between  them ;  it  was  exposed  for  many  days  to  the  sun's  light,  and  only 
assumed  a  faint  bluish  stain,  while  a  similar  piece  exposed  to  the  direct  rays  was  fully 
blackened  in  fifteen  minutes.  So  powerful  is  the  action  of  this  salt,  that  when  a  stratum 
of  it  not  more  than  the  hundredth  part  of  an  inch  thick  was  included  between  two 
plate  glasses,  it  stopped  the  decomposition  of  chloride  of  silver.  It  was  after  a  long 
examination  of  a  variety  of  substances  that  I  first  became  acquainted  with  the  great 
absorptive  power  of  the  chromates  of  potassa.  In  my  earlier  experiments,  T  had  made 
use  of  the  chloride  of  platinum  and  the  chloride  of  gold,  both  of  which  have  an  anal- 
ogous action.  The  solutions  which  I  have  recognised  as  possessing  this  power  in  the 
most  eminent  degree  are, 

Bichromate  of  potassa.  Muriate  of  iron. 

Chromate  of  potassa.  Chloride  of  gold. 

Yellow  hydro-sulphuret  of  ammonia.        Chloride  of  platinum. 

Hydro-sulphate  of  lime.  Coloured  vegetable  solutions. 

It  is  remarkable  that  all  the  mineral  solutions  on  this  list  are  yellow  ;  the  absorptive 
power,  however,  is  by  no  means  connected  with  that  colour,  for  the  yellow  tint  is  a 
compound  one  ;  all  the  rays  of  homogeneous  light  are  absorbed  by  one  or  other  of  the 
bodies  on  the  foregoing  list. 

411.  It  is  interesting  to  know  whether  these  absorptions  be  really  the  abstraction  of 
something  from  the  solar  ray,  or  merely  some  change  impressed  upon  it.  If  light 
consists  of  tremblings,  pulses,  or  undulations,  or  any  other  kind  of  motion  of  a  homo- 
geneous elastic  medium,  in  virtue  of  which  it  is  competent  to  excite  sensations  of  heat 
and  effect  chemical  changes,  we  might  explain  the  action  of  these  media  as  the  result 
of  some  change  occurring  to  that  motion,  either  in  direction  or  degree.  We  might 
suppose,  too,  that  when  a  ray  had  been  deprived  of  its  power  by  passage  through  one 
medium,  it  might  have  it  restored,  in  a  greater  or  less  degree,  by  being  transmitted 
through  another.    I  have  not  found,  in  thus  comparing  together  nearly  three  hundred 

O 


106 


DECOMPOSITION  OF  CARBONIC  ACID  BY  LEAVES. 


media,  any  indications  of  such  a  result,  and  therefore  suppose  that  something  of  a 
material  character  has  been  abstracted  from  the  ray  ;  that  it  is  really  a  loss,  and  not  a 
change. 

412.  Being  thus  possessed  of  the  means  of  depriving  the  beams  of  the  sun  of  their 
heat  and  their  chemical  force,  I  have  proceeded  to  examine  a  variety  of  questions 
of  interest.  A  great  many  changes  in  the  constitution  of  bodies,  on  their  exposure  to 
Ught,  are  recorded  in  the  books  of  chemistry  and  physics,  but  they  are  there  imputed 
to  light  in  the  aggregate,  without  any  reference  to  its  compound  character:  we  shall 
find  there  are  changes  due  to  the  colorific  ray,  changes  due  to  the  calorific  ray,  and 
changes  due  to  the  chemical  ray. 

413.  Decomposition  of  Carbonic  Acid  by  Leaves. — One  of  the  most  important 
and  extensive  functions  exercised  by  radiant  matter  from  the  sun  is  the  decomposition 
of  carbonic  acid  by  vegetable  leaves,  and  the  elimination  of  oxygeh  gas.  Vegetable  phy- 
siology looks  to  chemistry  for  information,  but  hitherto  the  chemist  has  not  possessed 
the  means  of  perfectly  developing  the  matter.  Its  intrinsic  importance  entitling  it  to 
investigation,  I  shall  not  offer  any  apology  for  passing  from  the  direct  object  of  this 
paper  to  the  mention  of  some  facts  necessary  to  the  thorough  understanding  of  the 
matter. 

414.  It  would  appear  that  there  is  a  particular  kind  of  combination,  to  which  attention 
has  hardly  yet  been  drawn,  distinct  from  what  is  understood  by  chemical  combination 
and  mechanical  mixture.  A  pint  of  alcohol  and  a  pint  of  water  being  mixed  together, 
the  result  will  measure  somewhat  less  than  a  quart,  and  the  same  might  be  indicated  of 
a  variety  of  other  liquids.  No  instance.  I  believe,  is  yet  on  record  of  a  like  penetration 
of  dimensions  being  observed  in  the  case  of  gases ;  if  it  exist  at  all,  it  exists  to  a  very 
small  amount,  and  the  change  of  volume  which  these  bodies  readily  experience,  by  al- 
teration of  temperature  and  pressure,  renders  so  minute  an  effect  very  difficult  of  detec- 
tion. It  has  been  supposed,  judging  from  analogy,  that  the  constituent  gases  of  the 
atmosphere,  the  uniting  volume  of  which  is  always  constant,  are  held  together  in  this 
manner,  or  that  the  whole  volume  is  condensed  and  retained  by  some  force  of  com- 
pression. There  are  some  experiments  whi(;h  indirectly  prove  this :  sound  passes 
along  different  media  with  different  velocities ;  if  a  cannon  were  therefore  discharged 
at  a  distance,  it  should  impress  the  ear  with  two  distinct  sounds ;  the  one  coming  along 
the  particles  of  nitrogen,  and  the  other  along  the  particles  of  oxygen.  But  it  is  well 
known,  from  observations  made  directly  on  this  point,  that  instead  of  there  being  any 
reduplication  of  the  sound,  it  comes  clear,  distinct,  and  alone ;  we  have,  therefore,  to 
infer  that  these  two  gases  are  held  together  in  a  state  of  compression. 

415.  In  making  experimental  investigations  on  this  matter,  two  different  courses  may 
be  followed  :  first,  we  may  measure  the  resulting  volume  after  the  mixture  of  known 
volumes  of  the  gases  under  trial ;  or,  secondly,  we  may  ascertain  whether  any  thermal 
disturbance  takes  place  during  the  act  of  their  uniting ;  the  latter  is  the  mode  I  have 
followed  in  my  researches. 

41G.  Take  a  cylindrical  glass  (A,  jig.  53)  two  inches  in  diameter  and  four  in  height ; 
close  its  upper  extremity  with  a  flat  piece  of  wood  by  means  of  cement;  in  the  centre 


I 


PENETRATION  OF  DIMENSIONS  OF  GASES. 


107 


of  it  cement  a  stopcock,  a,  of  large  bore;  and,  at  a  suitable  distance  from  that  centre, 
make  two  holes,  the  one  to  have  a  piece  of  bent  tube,  b,  cemented  into  it  to  serve  as  a 
gauge,  the  other  to  have  a  piece  of  copper  wire,  c,  bent  into  the  shape  (c,  Jig.  54), 
passed  through  it  air-tight,  by  means  of  a  cork,  x,  imbued  with  tallow.  The  other  ex- 
tremity of  the  cylindrical  glass  is  likewise  to  be  closed  by  a  flat  piece  of  wood  larger  than 
the  former,  for  the  purpose  of  bearing  a  little  cup,  d,  containing  coloured  water,  into 
which  the  gauge  tube  may  dip,  and  in  its  centre  it  is  to  be  perforated  to  admit  of  an 
arrangement,  as  in  Jig.  55,  where  is  a  perforated  cupping-glass,  having  a  stopcock, 
b,  mounted  on  it,  the  farther  extremity  of  which  opens  into  a  glass  pipe,  c,  wbich  termi- 
nates in  a  hole,  in  the  centre  of  a  flat  copper  circle,  a,  three  quarters  of  an  inch  in  diame- 
ter :  this  arrangement  is  to  be  cemented,  air-tight,  into  the  flat  piece  of  wood  that  closes 
the  lower  extremity  of  the  cylindrical  glass,  as  is  seen  in^o-.  53.  Moreover,  beneath 
the  cupping-glass  there  is  a  glass  reservoir,  g,  of  suitable  dimensions,  filled  with  water. 
The  object  of  this  arrangement  is  to  fill  a  soap  bubble  with  any  gas,  to  expose  it  to  at- 
mospheric air,  to  burst  it  at  pleasure,  and  to  mark  any  thermal  expansion  of  the  two 
gases  by  the  indications  of  the  gauge  ;  the  mode  in  which  this  is  accomplished  will  be 
described  in  the  following  illustration. 

417.  The  whole  apparatus  having  stood  for  some  time  in  a  quiet  room,  along  with 
the  gases  to  be  tried,  until  they  have  all  acquired  a  uniform  temperature,  close  the 
lower  cock,  fill  the  cupping-glass  with  hydrogen  gas,  and  raise  the  reservoir,  g,  so  that 
the  level  of  the  water  may  be  near  the  top  of  the  cupping-glass.  The  upper  cock,  a, 
being  open,  convey  through  its  bore,  by  means  of  a  glass  tube  of  smaller  diameter,  a 
little  soap-water,  which  is  to  be  deposited  on  the  copper  circle  in  its  centre,  over  where 
the  glass  pipe,  e,  opens;  the  tube  is  then  withdrawn.  Next  open  slowly  the  lower  cock, 
and  as  the  gas  is  expelled  from  the  cupping-glass  by  the  pressure  of  the  water  in  the  res- 
ervoir, it  expands  a  bubble  in  the  large  cylinder,  the  displaced  atmospheric  air  passing  out 
through  the  upper  cock.  When  this  bubble  has  attained  the  dimensions  desired,  close 
both  cocks,  and  observe  if  the  liquid  in  the  gauge  be  stationary;  if  so,  turn  the  wire  c 
on  its  axis,  so  as  to  bring  its  crooked  extremity,  which  is  within  the  cylinder,  in  con- 
tact with  the  bubble;  it  bursts,  there  is  a  thermal  disturbance,  and  an  expansion  of  the 
two  gases,  for  the  fluid  in  the  gauge  instantly  falls,  and  as  the  gases  cool,  it  slowly  re- 
turns to  its  former  position.  If  a  bubble  of  atmospheric  air  be  employed  instead  of  a 
bubble  of  hydrogen,  these  effects  will  not  ensue.  We  therefore  conclude,  that  when 
hydrogen  gas  is  mixed  with  atmospheric  air,  the  temperature  suddenly  rises,  and  there- 
fore that  it  is  probable  that  the  volume  of  the  mixture  is  less  than  the  sum  of  the  volume 
of  its  integrant  constituents. 

418.  If  a  soap  bubble,  filled  with  hydrogen,  be  burst  in  an  atmosphere  of  nitrogen 
gas,  which  may  be  effected  by  using  a  more  complex  arrangement  than  that  indicated 
in  the  preceding  section,  there  is  also  a  thermal  expansion,  indicating  that  the  constit- 
uents of  ammoniacal  gas,  even  without  chemically  uniting  one  with  another,  exercise 
an  attraction  for,  or  a  pressure  on,  each  other,  a  kind  of  capillary  action.  These  com- 
pounds, for  they  form  a  distinct  class  of  bodies,  a  class  by  no  means  of  small  extent,  re- 
quire a  distinct  name.    I  have  suggested  that  of  capillary  compounds,  because  they 


108 


DECOMPOSITION  OF  CARBONIC  ACID  IN  LIGHT  OF  DIFFERENT  COLOURS. 


exist  under,  and  can  be  separated  by,  the  force  of  capillary  attraction.  An  example 
will  here  illustrate  what  is  meant.  Oxygen  and  hydrogen  gases  may  be  mingled  with 
each  other  in  the  proportion  of  one  to  two,  the  result  existing  in  a  compressed  state, 
and  forming  a  capillary  compound  ;  the  contact  of  flame,  or  the  passage  of  an  electric 
spark,  changes  it  into  aqueous  gas,  a  chemical  compound  ;  in  the  former  state,  de- 
composition is  re;idily  effected  by  capillary  attraction;  in  the  latter  it  cannot  produce 
such  a  result.  The  general  law  of  these  decompositions  by  tissues  without  pores  of 
sensible  size  is  given  in  Chapter  IV. ;  it  is  a  very  simple  one,  showing  that  a  capillary 
equilibrium  is  gained  only  when  the  composition  of  gaseous  media  on  each  side  of  a 
barrier  is  chemically  the  same.  This  was  proved  by  exposing  extremely  thin  soap  bub- 
bles, filled  with  different  gases,  to  different  gaseous  atmospheres,  and  then  measuring 
and  analyzing  the  media  within  the  bubble  and  without.  This  law  is  applicable  not 
only  where  a  barrier  separates  a  gas  from  a  gas,  but  also  when  one  of  the  gases  is  held 
in  solution  by  water;  and  in  the  energy  with  which  the  media  endeavour  to  attain  an 
equilibrium,  is  to  be  found  not  only  one  of  the  causes  of  the  decomposition  of  carbonic 
acid  by  the  light  of  the  sun,  but  also  a  very  fruitful  source  of  erroneous  experimenting. 
Having  made  reference  to  this  matter,  I  proceed  to  detail  the  steps  which  have  been 
taken  to  illustrate  this  decomposition. 

419.  Take  four  globular  vessels,  such  as  a  (Jig.  56),  three  or  four  inches  in  diame- 
ter, with  necks  a  couple  of  inches  long;  fill  them  with  spring  water,  and  put  a  bunch 
of  pine  leaves  in  it ;  innnerse  the  end  of  the  neck  beneath  the  surface  of  the  mercury 
contained  in  a  cup,  b.  Let  one  of  these  vessels,  designated  A,  be  exposed  to  the  sun's 
direct  ray ;  a  second,  B,  to  the  light  which  has  passed  through  a  solution  of  bichro- 
mate of  potassa;  a  third,  C,  to  the  light  which  has  passed  through  a  solution  of  sul- 
phate of  copper  and  ammonia  ;  and  a  fourth,  D,  in  a  dark  place.  It  will  be  found  that 
in  the  course  of  a  few  hours,  A  has  eliminated  most  gas,  B  somewhat  less,  and  C  and  D 
none  at  all ;  this  is  a  very  instructive  experiment ;  we  find  from  A  and  D  that  the  sun's 
rays  have  the  power  of  eliminating  gas  from  solutions  ;  from  B  we  learn  that  the  ab- 
sence of  the  chemical  rays  does  not  affect  the  apjmrent  result,  but  that  if  the  calorific 
rays  are  obstructed,  it  ceases  to  go  on. 

420.  A  variety  of  experiments  having  thus  convinced  me  that  the  mere  evolution  of 
gas  is  neither  due  to  the  rays  of  light,  nor  to  the  chemical  rays,  I  have  attempted  to 
produce  a  like  effect  with  the  calorific  rays  emitted  from  a  common  fire ;  rays  in  which 
the  light  was  altogether  disproportioned  to  the  heat,  and  the  chemical  power  totally 
wanting.  The  arrangement  is  as  follows:  in  the  focus  of  a  concave  speculum  of  brass 
eighteen  inches  in  diameter,  I  placed  one  of  the  glass  globes  of  the  preceding  section, 
so  that  it  might  receive  the  rays  emitted  from  a  common  wood  fire,  converged  on  it  by 
the  mirror.  The  fire  was  burning  without  flame,  being  what  is  technically  called  a 
dead  fire,  and  the  distance  of  the  mirror  eight  feet.  In  a  few  moments  gas  was  copi- 
ously liberated,  more  copiously  than  if  it  had  even  been  exposed  to  the  solar  ray.  In 
Jig.  57,  this  arrangement  is  depicted :  a  is  the  concave  mirror,  h  the  glass  matrass  filled 
with  spring  water,  and  containing  a  bunch  of  pine  leaves,  c  a  cup  of  mercury  into  which 
its  neck  dips. 


GAS  FROM  LEAVES  CONTAINS  NITROGEN.  109 

421.  I  shall  have  occasion  to  show,  hereafter,  that  when  a  beam  of  light  falls  upon 
any  surface  in  contact  with  a  medium,  it  causes  that  surface  to  exert  an  apparent  pres- 
sure on  the  medium,  capable,  at  times,  of  producing  singular  effects ;  it  is,  therefore, 
probable  that  to  this  action  we  are  to  attribute  the  evolution  of  gas  by  vegetable  leaves, 
spun  glass,  raw  silk,  &c.  The  percolation  of  liquids  and  gases  through  tissues,  in  obe- 
dience to  the  laws  of  capillary  attraction,  should  also,  on  these  principles,  be  controlled 
by  the  action  of  a  solar  beam.  If  we  arrange  two  Champagne  glasses  {fg.  58),  with 
their  footstalks  cut  off,  each  capped  with  a  thin  lamina  of  India-rubber,  their  narrow 
apertures  dipping  into  cups  of  water,  so  that  they  may  be  in  all  respects  as  like  each 
other  as  possible,  and  fill  them  with  protoxide  of  nitrogen,  we  shall  find  that  one  of 
them  exposed  to  the  sunbeam  will  throw  off  its  gas  much  quicker  than  the  other,  shut 
up  in  the  dark.  Or,  if  one  of  them  be  exposed  to  an  atmosphere  much  warmer  than 
the  other,  the  liquid  confining  the  gas  rises  more  rapidly.  It  has  been  remarked  that 
the  experiment  of  which  I  gave  an  account  (52),  of  the  passage  of  hydrogen  gas  through 
a  thin  film  without  pores  of  sensible  size,  is  not  uniformly  attended  with  success.  In 
examining  the  causes  of  failure,  I  have  been  able  to  trace  them  entirely  to  this  source ; 
at  a  certain  temperature  the  effect  is  scarcely  perceptible,  but  as  the  thermometer  rises 
it  becomes  more  and  more  marked.  The  same  observations  may  be  made  of  annnonia- 
cal  vapour.  There  are  temperatures  at  which  these  permeations  are  imperceptible,  but 
at  75°  Fah.  they  take  place  with  great  rapidity. 

422.  Radiant  heat,  whether  of  the  sun  or  of  terrestrial  fire,  impinging  on  the  surface  of 
an  obstacle,  causes  it  to  exert  an  increased  action,  resembling  a  force  of  attraction  or 
pressure,  on  any  medium  with  which  it  may  be  in  contact.  A  few  fibres  of  unspun 
silk  being  immersed  in  water  containing  the  elements  of  atmospheric  air  in  solution,  and 
exposed  to  the  sunshine,  became  speedily  covered  with  bubbles  of  gas.  The  exact 
chemical  constitution  of  these  bubbles  is  determined  by  a  variety  of  circumstances:  by 
the  velocity  of  evolution,  by  the  solvent  action  of  the  water,  which  is  greater  for  one 
gas  than  another,  and  by  the  presence  or  absence  of  the  chemical  rays.  I  shall  here 
be  excused  for  remarking  a  circumstance  which  appears  to  me  indicative  of  a  proneness, 
even  in  capillary  compounds,  to  exhibit  tendencies  of  combination  by  multiple  volumes. 
Atmospheric  air  contains  oxygen  and  nitrogen  in  the  proportion  of  1  to  4;  the  gas  ex- 
pelled from  spring  water  contains  the  same  elements  in  the  proportion  of  1  to  2  ;  and 
the  gas  given  off  by  pine  leaves  from  water,  holding  carbonic  acid  in  solution,  con- 
tains the  same  elements  in  the  proportion  of  2  to  1. 

423.  The  chemical  rays  emitted  from  the  sun  are  not,  therefore,  the  cause  of  the 
evolution  of  gas  from  liquids  by  fibres,  or  by  vegetable  leaves,  for  it  takes  place  in  their 
absence ;  the  blue,  the  indigo,  and  the  violet  rays  have  nothing  to  do  with  it  for  the 
same  reason  ;  and  the  green,  yellow,  orange,  and  red  are  not  the  cause  of  it,  for  though 
they  are  present,  it  refuses  to  go  on.  To  the  calorific  ray  we  are,  therefore,  to  impute 
it.  It  happens  not  by  the  action  of  any  kind  of  light  operating  as  a  mere  stimulus 
on  plants,  for  when  the  light  is  nearly  absent,  it  goes  on  with  undiminished  energy. 

424.  The  evolution  of  gas  depending,  therefore,  on  the  rays  of  heat,  we  are  next  led 
to  inquire  whether  the  chemical  rays  affect  the  operation  in  any  manner.    To  determine 


110 


GAS  FROM  LEAVES  CONTAINS  NITROGEN. 


this,  I  exposed  a  quantity  of  boiled  water,  which  had  been  suffered  to  cool  in  vacuo, 
to  carbonic  acid  gas,  of  which  it  absorbed  a  certain  amount.  A  portion  of  this  water 
was  placed  in  the  focus  of  the  brass  mirror,  and  was  there  acted  upon  by  the  non-lumi- 
nous rays;  its  temperature  never  exceeded  140°  Fah.  In  a  short  time  the  pine  leaves 
commenced  giving  off  gas  very  copiously,  and  continued  to  do  so ;  but  it  was  found, 
on  trial,  that  nearly  the  whole  of  it  was  absorbed  by  hme-water,  and  that  no  decom- 
position had  occurred.  Therefore,  though  rays  of  non-luminous  heat  are  competent  to 
cause  the  evolving  of  gas,  they  are  not  able  to  cause  decomposition. 

425.  The  record  of  an  analysis  will  place  this  effect  in  its  true  hght :  care  being 
taken  that  the  water  should  be  impregnated  with  pure  carbonic  acid  gas,  and  the  leaves 
recent,  when  a  sufficient  quantity  was  evolved,  39  measures  were  taken,  of  which  caustic 
potassa  absorbed  34.  Hydrogen  gas  being  then  added,  a  diminution  to  the  amount  of  4 
volumes  was  produced  by  a  platinum  ball;  the  remaining  gas  proved  to  be  nitrogen. 
The  composition  of  this  gas  was,  therefore, 

Carbonic  acid  34-00 

Nitrogen  3-67 

Oxygen    .......  1-33 

39-00 

It  is  proper  to  observe  that  a  change  very  evidently  takes  place  in  the  structure  of  the 
vegetable  leaves,  their  colour  becoming  of  a  dirty  brown,  and  their  greenness  is  lost. 
Whether  it  is  a  change  of  their  acting  tissue  which  hinders  decomposition,  or  whether 
there  is  some  peculiarity  in  the  constitution  of  non-luminous  heat,  which  incapacitates 
it  from  producing  tbose  effects  which  result  from  caloric  radiating  from  highly  incan- 
descent bodies,  I  shall  proceed  to  discuss. 

426.  Let  us  first  consider  what  is  the  action  of  an  ordinary  unchanged  sunbeam  on  car- 
bonic acid  in  solution,  and  in  contact  with  vegetable  matter.  A  wide  distinction  is 
here  to  be  made  between  common  spring  water,  such  as  pump  water,  and  water  charged 
with  carbonic  acid  oyily ;  the  former  contains  a  compound  of  oxygen  and  nitrogen, 
isomeric  with  protoxide  of  nitrogen  ;  but  the  protoxide  is  a  chemical  compound,  having 
its  two  volumes  of  nitrogen  compressed  into  one,  while  this  is  a  capillary  compound, 
existing  with  an  almost  insensible  condensation.  The  processes  of  evolving  gas  from 
spring  water  and  from  carbonated  water  are  essentially  different ;  the  former  taking 
place  by  an  exaltation  of  temperature  occasioned  by  the  impinging  of  radiant  heat,  no 
kind  of  decomposition  at  all  going  on ;  but  the  latter  is  accompanied  by  a  true  decom- 
position, due  to  the  presence  of  vegetable  matter. 

427.  This  case  will  be  better  understood  by  an  analysis  of  the  gas  given  off  from 
carbonated  water.  A  certain  volume  of  water  had  its  carbonic  acid  and  all  other  gas- 
eous impurities  expelled  by  long-continued  boiling ;  it  was  then  rapidly  cooled  by  refri- 
geratory processes,  and  impregnated  with  pure  carbonic  acid  gas  ;  being  introduced 
into  a  matrass  (,fig.  56)  with  a  bunch  of  pine  leaves,  the  neck  of  the  matrass  dipping 
under  the  surface  of  some  mercury  contained  in  a  cup,  so  as  to  cut  off  communication 
with  the  atmosphere,  it  was  exposed  to  the  sun,  the  day  being  very  favourable,  clear 
and  hot;  47-50  measures  of  the  gas  evolved  were  taken  ;  a  piece  of  caustic  potassa  ab- 


GAS  FROM  LEAVES  CONTAINS  NITROGEN. 


Ill 


sorbed  3-50  measures  of  carbonic  acid,  the  remainder  being  44-00  measures ;  90  measures 
of  hydrogen  were  added  thereto,  making  the  full  volume  134-00  measures;  a  platinum 
ball  reduced  this  to  67-00  ;  indicating  22-33  of  oxygen,  there  remaining  of  nitrogen 
21-67.    The  composition  of  this  gas  was,  therefore, 


Oxygen  2233 

Nitrogen  21-67 

Carbonic  acid  3-50 

47-50 


To  prove  that  the  remainder  here  spoken  of  was  really  nitrogen,  one  hundred  volumes 
of  the  original  gas  were  taken,  and  the  electric  spark  passed  through  it ;  there  was  no 
diminution  in  the  volume,  nor  any  carbonic  acid  gas  generated ;  it  could  not,  therefore, 
be  carbonic  oxide,  hydrogen  gas,  nor  any  of  the  carburets  of  hydrogen  ;  it  possessed, 
moreover,  all  the  negative  qualities  of  nitrogen. 

428.  But  the  solution  was  composed  of  carbonic  acid  and  water,  great  care  having 
been  taken  to  cut  off  all  access  of  the  atmosphere  during  its  preparation,  and  also  during 
its  exposure  to  the  sun,  for  fear  of  capillary  interchange  of  the  carbonic  acid  with  the 
gaseous  elements  of  atmospheric  air.  None  such  had  occurred.  From  what  source, 
then,  came  the  large  amount  of  nitrogen  gas  evolved  1  the  only  elements  within  the 
matrass  were  carbon,  hydrogen,  and  oxygen,  yet  here  a  large  amount  of  nitrogen  was 
found,  which  could  have  come  from  no  other  source  than  the  pine  leaves. 

429.  In  this  experiment  the  pine  leaves  absorbed  one  measure  of  carbonic  oxide,  and 
gave  in  exchange  for  it  one  measure  of  nitrogen,  and  the  resulting  gas  contained,  there- 
fore, half  its  volume  of  nitrogen,  and  half  of  oxygen,  mixed  without  sensible  con- 
densation. 

430.  Hitherto  it  has  often  been  asserted  by  chemists,  that  when  vegetable  leaves  were 
placed  in  carbonated  water,  they  absorbed  the  carbon,  and  caused  the  oxygen  to  be 
evolved.  Vegetable  physiologists,  botanists,  and  others,  have  raised  a  great  many  the- 
ories upon  this  fact,  which,  however,  a  long  course  of  experiments  assures  me  are  with- 
out any  foundation.  There  is  no  truth  in  the  idea  that  plants  absorb  carbonic  acid, 
and  assimilate  carbon  and  evolve  oxygen.  On  the  contrary,  they  actually  evolve  nitro- 
gen, and  the  decomposition  of  carbonic  acid,  though  remotely  brought  about  by  the 
action  of  the  solar  ray,  is  mainly  due  to  the  complex  play  of  affinities  of  the  elementary 
constituents  of  the  plants. 

431.  I  will  here  give  another  example  in  point,  substantiating  the  same  fact  under 
different  circumstances.  Carbonated  water  that  had  been  exposed  with  due  care  to 
the  sun  for  two  days  being  provided,  25-75  measures  of  the  resulting  gas  were  taken, 
and  found  to  contain  1-25  of  carbonic  acid,  for  caustic  potassa  diminished  them  to 
24.50.  Next,  31-50  measures  of  hydrogen  were  added,  making  in  all  56-00,  and  a  pla- 
tinum ball  being  introduced,  there  remained  7-50,  indicating  16-16  volumes  of  oxygen 
and  8-34  of  nitrogen,  the  composition  of  the  gas  being,  therefore 

Oxygen  16-16 

Nitrogen  8*34 

Carbonic  acid   .    .    .    .  1*55 

25-75 


112 


CHEMICAL  RAYS  OF  DIFFERENT  COLOURS. 


Allowing  for  unavoidable  errors  of  manipulation,  the  resulting  gas  was,  therefore,  one 
third  nitrogen  and  two  thirds  oxygen,  united  without  sensible  condensation. 

432.  If  any  farther  proof  was  required  that  the  evolution  of  nitrogen  by  the  plant  is 
an  essential  part  of  this  decomposition,  it  is  furnished  by  the  results  of  an  experiment 
in  which  spun  glass  was  used  to  replace  the  pine  leaves.  This  arrangement,  though 
exposed  to  the  sun  under  the  most  favourable  circumstances,  will  not  evolve  any  gas, 
but  on  passing  into  it  a  leaf,  no  matter  how  small,  decomposition  at  once  commences, 
because  the  requisite  quantity  of  nitrogen  is  given  olf. 

433.  A  box,  «,  h,  e,  e,  of  a  cubical  shape  {fig.  69),  and  nearly  12  inches  in  each  of  its 
dimensions,  had  one  of  its  sides  taken  out  and  replaced  by  a  trough,  k  k,  of  suitable  size, 
consisting  of  two  glass  plates  cemented  at  a  distance  of  one  fourth  of  an  inch  from  each 
other.  This  trough  was  filled  with  a  solution  of  bi-chromate  of  potassa  ;  one  of  the 
sides  of  the  box  was  hung  on  hinges,  e  e,  as  a  door,  for  the  sake  of  obtaining  access  to 
the  interior.  Within  this  little  chamber,  a  matrass  filled  with  carbonated  water,  and 
enclosing  a  bunch  of  pine  leaves,  its  neck  dipping  beneath  the  surface  of  some  water 
in  a  cup,  was  shut  up  and  exposed  to  the  sun's  rays,  which,  passing  through  the  trough, 
impinged  upon  it.  In  a  short  time  air-bubbles  were  copiously  given  oflf,  and  when  a 
sufficient  quantity  was  obtained  for  analysis,  its  constitution  was  determined.  The  fol- 
lowing is  selected  from  a  number  of  analyses,  being  probably  the  most  correct,  and  very 
nearly  the  mean : 

Carbonic  acid    ....  16-00 

Oxygen  8-16 

Nitrogen  4-84 

29-00 

We  here  remark  the  existence  of  a  far  larger  proportion  of  carbonic  acid,  but  the  rela- 
tive proportion  of  the  oxygen  and  nitrogen  is  still  observed  with  tolerable  accuracy  ;  the 
deviation  may  be  satisfactorily  referred  to  disturbing  causes.  The  greater  amount  of 
carbonic  acid,  as  compared  with  sections  (427)  and  (431),  may  likewise  be  due  to  the 
higher  temperature  of  the  arrangement  when  shut  up  in  a  close  box,  where  currents  of 
air,  or  other  cooling  agents,  could  not  have  free  access  to  it.  Or  it  may  hereafter  be 
found  that  there  are  chemical  rays  of  different  colours,  as  it  were  ;  or,  more  strictly,  of 
different  refrangibility  and  absorbability,  and  that  those  which  find  a  passage  through  bi- 
chromate of  potassa  can  cause  the  decomposition  of  carbonic  acid,  though  they  cannot 
blacken  chloride  of  silver.  The  doctrine  that  chemical  rays  are  nothing  more  than  un- 
dulations of  an  elastic  medium,  the  waves  of  which  vary  in  breadth,  I  shall  endeavour 
to  support ;  each  of  these  kind  of  waves  is  competent  to  bring  about  changes  peculiar  to 
itself.  Not  in  this  place,  however,  to  anticipate  what  I  have  to  offer  on  these  matters, 
I  shall  continue  to  use  the  term  chemical  rays  as  expressing  those  which  blacken  chlo- 
ride of  silver ;  and  these,  I  say,  are  not  engaged  in  the  decomposition  of  carbonic  acid. 

434.  From  the  first  observations  made  on  the  decomposition  of  carbonic  acid  by 
Priestley,  this  subject  has  afforded  much  scope  for  chemical  speculation.  Count  Rum- 
ford  examined  it  successfully,  but  wanting  means  of  accurate  gaseous  analysis,  and, 
above  all,  not  understanding  the  doctrine  and  laws  of  interchange  through  tissues,  his 


RELATION  OF  VOLUME  IN  THE  GASES  ABSORBED  AND  EVOLVED. 


113 


conclusions  are  devoid  of  that  degree  of  precision  which  the  advance  of  chemistry,  in 
all  its  departments,  enables  us  to  attain.  The  conclusion  to  which  these  earlier  phi- 
losophers came  was,  that  plants  had  the  power  of  absorbing  carbonic  acid  from  the  air, 
and  rendering  oxygen  in  return  by  elaboration  from  their  vessels ;  and  this  they  regard- 
ed as  the  great  means  employed  by  nature  to  maintain  the  integrity  of  the  composition 
of  the  atmosphere.  A  similar  view  has  been  taken  of  this  subject  by  almost  every 
philosopher  who  has  since  examined  it.  Professor  Burnet,  to  accommodate  the  the- 
ory to  the  observed  facts,  infers  that  plants  exercise  two  functions,  the  one  of  breathing, 
the  other  of  digestion,  the  latter  only  occurring  during  the  stimulant  action  of  the  sun- 
shine. This  phenomenon  is,  however,  unquestionably,  one  depending  on  the  exalted 
capillary  action  of  a  tissue  when  radiant  matter  impinges  on  it ;  and  the  evolution  of 
nitrogen,  or  of  some  other  gaseous  or  vaporous  matter,  is,  therefore,  an  essential  part  of 
the  process. 

435.  The  analyses  made  in  the  foregoing  sections  show  that  the  volume  of  gas 
which  remains  after  action  is  complete,  is  exactly  the  same  as  the  volume  of  carbonic 
acid  first  operated  on.  The  best  method  of  proving  this  directly  is  to  take  a  tube,  the 
diameter  of  which  may  be  half  an  inch  or  upward,  which  is  graduated  into  inches  and 
decimal  parts.  Fill  it  with  water,  from  which  all  gaseous  matter  has  been  expelled  by 
long-continued  boiling ;  place  a  few  vegetable  leaves  in  it,  carefully  removing  any  bub- 
bles of  air  which  may  be  attached  to  them ;  invert  the  tube  in  a  vessel  of  water,  and 
pass  into  it  as  quickly  as  possible  a  measured  quantity  of  pure  carbonic  acid,  and  trans- 
fer it  to  a  mercurial  trough.  This  arrangement  is  seen  jig.  60.  Conduct  the  experi- 
ment first  in  a  cool,  dark  place  ;  absorption  will  rapidly  go  on,  and  in  a  short  time  all, 
or  the  greater  part  of  the  carbonic  acid  will  disappear,  a  column  of  mercury,  e  e,  rising 
in  the  tube  to  replace  the  gas.  It  is  to  be  remarked  that  it  is  not  always  easy  to  pro- 
cure the  entire  absorption  of  all  the  gas,  a  little  bubble  remaining  in  the  upper  part  of 
the  tube,  containing  the  impurities  that  may  have  existed  in  the  gas,  and  also  any  re- 
mains of  the  carbonic  acid,  for  the  amount  absorbed  depends  upon  several  circumstan- 
ces, as  the  relative  proportion  of  the  volume  of  gas  to  the  volume  of  water,  the  height 
of  the  mercurial  column  suspended  in  the  tube,  the  temperature  of  the  arrangement,  &c. 
Then,  on  exposure  to  the  solar  rays,  gas  is  copiously  given  ofi',  the  quantity  continually 
decreasing  until  farther  exposure  ceases  to  evolve  any  more.  On  makinii  the  usual 
corrections  for  temperature  and  pressure,  the  aggregate  of  evolved  gas  will  be  found 
precisely  the  same  as  the  volume  first  operated  on. 

436.  Sometimes,  however,  the  volume  is  increased  by  an  amount  varying  from  -10 
downward,  due  chiefly  to  a  certain  amount  of  gas  given  off"  from  the  leaves  extrane- 
ously,  and  partly  to  the  capillary  action  of  the  whole  system  upon  the  elements  of  at- 
mospheric air,  which  are  transferred  by  slow  degrees  to  the  water  operated  upon,  should 
there  be  a  film  of  that  fluid  between  the  mercury  and  the  sides  of  the  glass  tube  ;  but, 
by  making  allowance  for  these  disturbing  actions,  the  proportion  of  equality  will  be 
found  to  be  rigidly  observed  by  the  absorbed  and  the  evolved  quantities. 

437.  We  find,  therefore,  that  the  evolution  and  decomposition  of  carbonic  acid  by 
the  solar  ray  are  due  to  that  part  of  it  exciting  heat ;  that  the  chemical  ray  has  no  di- 


114 


CAUSE  OF  THE  PHENOMENON. 


rect  agency  in  the  matter ;  it  may  bring  about  clianges  which,  to  a  certain  extent, 
complicate  the  phenomenon,  but  it  does  not  produce  the  abstraction  of  any  compound 
of  oxygen  and  carbon  from  carbonic  acid.    Apart  from  the  agencies  exercised  by  the 
elements  of  the  plant,  agencies  which  are  unquestionably  of  the  utmost  importance,  the 
decomposition  is  remotely  brought  about  by  the  action  of  radiant  matter.    But  non- 
luminous  heat,  though  capable  of  evolving  gas,  produces  no  change  of  its  constitution  ; 
shall  we,  then,  suppose  that  there  is  a  difference,  in  point  of  quality,  between  the  heat 
given  off  by  the  bodies  below  ignition,  and  the  heat  of  incandescent  matter  1    Or  does 
the  light  itself  aid  decomposition  ?    An  experiment  may  be  made  which  appears  to  me 
to  bear  directly  on  the  answer  which  should  be  given  to  this  query.    Let  a  beam  of 
light  (^fig.  61),  two  inches  in  diameter,  pass  through  an  aperture  in  the  shutter  A  B,  and 
fall  upon  any  medium,  c  d,  which  absorbs  a  certain  number  of  the  rays  of  heat,  as 
bichromate  of  potassa,  which  may  be  so  diluted  as  to  absorb  exactly  50  rays  out  of  every 
100.    Having,  by  means  of  a  good  thermometer,  g,  measured  this,  let  the  beam  of  light 
pass  through  a  second  trough,  e  f,  containing  the  same  solution  of  the  same  strength,  and 
its  temperature  again  be  taken,  it  will  appear  that  the  ray,  instead  of  losing  half  its 
heat,  will  contain  nearly  all  of  it;  or,  in  other  words,  the  second  trough  exerts  no  action 
on  the  passing  beam.    In  an  experiment  tried  after  the  manner  here  indicated,  the  ther- 
mometer having  shown  a  loss  of  50  rays  by  the  action  of  the  first  trough,  fell  only  to 
47,  or  gave  a  loss  of  three  rays  only  as  the  action  of  the  second  trough;  an  action  to  be 
referred,  undoubtedly,  to  a  degree  of  turbidness  which  does  exist,  to  a  small  extent,  in 
the  clearest  solutions ;  and  also  to  the  reflective  and  scattering  action  of  the  surfaces  of 
the  troughs.    Now  the  very  same  thing  takes  place  in  the  case  of  light.    A  beam  that 
has  passed  through  a  green,  or  any  other  coloured  glass,  loses  much  of  its  intensity,  but 
if  it  pass  through  a  second  plate,  of  the  same  tint,  the  second  loss  is  entirely  dispropor- 
tionate to  the  former ;  and  the  reason  of  this  is  very  apparent,  for  if  the  second  plate  had 
been  of  a  different  colour,  the  ray  might  have  been  much  more  affected,  or  even  entirely 
extinguished.    Delaroche  made  an  identical  observation  in  the  case  of  non-luminous 
heat,  for  he  proved  that  a  plate  of  glass  obstructed  a  large  portion  of  the  rays  falling 
on  it,  but  that  a  second  plate  allowed  these  rays  to  pass  with  far  less  loss.  Now 
these  experiments  would  lead  us  to  conclude  that  there  are  essential  differences  in  ra- 
diant heat  analogous  to  the  differences  in  light.    The  rays  of  heat  given  off  by  a  can- 
ister of  hot  water  may  be,  to  use  an  expressive  solecism,  violet  heat,  and  a  piece  of 
transparent  glass  may  be  able  to  transmit  green  heat  only ;  hence,  in  using  two  plates, 
the  absorptive  action  of  the  first  has  the  largest  share  in  producing  the  phenomenon, 
the  second  transmitting  nearly  all  which  passed  the  first,  an  action  identical  with  that 
of  coloured  glass  on  light.    Bodies,  as  their  temperature  rises,  emit  more  and  more  rays 
capable  of  passing  through  glass,  simply  because  they  become  of  that  class  over  which 
the  medium  docs  not  exercise  an  absorptive  power. 

438.  The  general  conclusion  which  we  are  to  draw  from  these  researches  is,  that 
the  decomposition  of  carbonic  acid  gas  by  vegetable  matter  is  a  very  complex  phenom- 
enon, due  to  the  combined  action  of  three  forces  :  1st,  the  decomposing  action  of  a  tissue; 
2d,  to  the  impinging  of  radiant  heat ;  3d,  to  chemical  affinity,  it  being  probable  that  any 


/ 

NON-OXYGENATION  OF  PHOSPHORUS.  115 

of  these  alone  would  be  incompetent  to  produce  this  result.  And  in  the  case  of  gas, 
such  as  oxygen,  being  evolved  from  spring  water,  we  are  to  refer  the  change  to  the 
ready  decomposition  of  capillary  compounds,  compounds  essentially  distinct  from  chem- 
ical, and  which  can  suffer  decomposition  by  the  force  of  capillary  action.  The  colo- 
rific and  the  chemical  rays  have  no  influence  in  this  latter  case. 

439.  NoiN-oxYGENATioN  OF  Phosphorus. — It  is  stated  in  the  books  that  Ritter,  in 
making  observations  on  the  slow  combustion  of  phosphorus  at  common  temperatures, 
found  that  it  emitted  white  fumes  in  the  invisible  red  ray  of  the  solar  spectrum,  hut  in 
the  invisible  violet,  phosphorus  in  a  state  of  oxygenation  was  instantly  extinguished. 
As  a  similar  action  is  alleged  to  take  place  when  the  sun's  rays  shine  on  ignited  car- 
bon, it  becomes  desirable  to  understand  the  mode  of  action  :  the  original  experiment  of 
Ritter  was  therefore  repeated,  with  a  view  to  ascertain  its  accuracy.  A  cylinder  of 
phosphorus,  a  h  {Jig.  62),  an  inch  long,  and  about  one  sixth  of  an  inch  in  diameter, 
was  shielded  from  the  action  of  aerial  currents  by  a  glass  jar.  In  front  of  the  jar  an 
equiangular  prism  of  flint  glass  was  placed,  so  that  the  rays  of  a  decomposed  beam  of 
light  coming  in  through  the  shutter,  cd,  could  successively  be  thrown  on  the  phosphorus, 
which  was  placed  horizontally  in  the  jar  ;  the  beam  of  light  also  came  nearly  horizon- 
tally into  the  room,  reflected  by  the  arrangement  already  described.  Situated  thus, 
by  turning  the  prism  on  its  axis,  any  ray  could  be  made  to  cover  the  phosphorus:  the 
temperature  in  the  shade  being  72°  Fahr.,  a  fine  sheet  of  metaphosphoric  acid,  mingled 
with  vapour  of  phosphorus,  so  thin  as  to  be  almost  imperceptible,  except  in  certain  po- 
sitions, was  observed  to  be  rising  from  the  cylinder;  sometimes  it  would  form  a  fine 
cloud,  which  hung  for  a  moment  on  the  phosphorus,  and  then  rose  gracefully  in  curled 
wreaths.  The  extreme  mobility  of  this  cloud  was  remarkable  :  even  the  warmth  of  the 
observer,  by  causing  currents  within  the  jar,  would  affect  it ;  if  the  hand  approached,  as 
at  A  {fig.  63),  the  phosphoric  vapour  came  to  the  side  of  the  vessel,  as  it  were  to  meet 
it,  and  then  rebounded  and  circulated  along  the  top  of  the  jar.  The  size,  position, 
and  shape  of  this  cloud,  when  enveloped  in  the  red  light  of  the  prism,  were  deliber- 
ately marked;  its  motions  were  merely  more  capricious  than  when  in  the  shade.  And 
now,  by  turning  the  prism,  the  extreme  violet  ray  was  brought  upon  it,  but  neither 
did  its  motion,  nor  magnitude,  nor  figure  appear  in  any  wise  changed. 

440.  The  impression  conveyed  by  Ritter's  experiment  is,  that  the  chemical  rays 
possess  the  faculty  of  hindering  oxygenation.  The  negative  conclusion  here  arrived 
at  might  be  due  to  local  circumstances,  and  be  referred  to  the  action  of  the  prism,  as 
to  its  composition,  to  the  state  of  the  atmosphere,  &c.;  but  no  better  success  attended 
a  variety  of  trials  made  on  different  days,  and  with  prisms  of  crown  glass,  turpentine, 
and  water.  Trial  was  therefore  made  of  absorbing  media,  a  beam  of  light  being  made 
to  pass  at  one  time  through  a  solution  of  sulphate  of  copper  and  ammonia,  and  at  an- 
other through  bichromate  of  potassa,  but  the  condition  of  the  phosphorescent  cloud 
was  found  to  be  too  rough  an  estimate  of  the  real  action.  A  cylinder  of  glass  (A  B, 
fig.  64),  -75  inch  in  diameter  and  3  inches  long,  was  therefore  fitted  at  its  upper  end 
with  a  stopcock,  a;  its  lower  extremity  was  closed  air-tight  with  a  cork,  through  which 
an  inverted  siphon,  h  b,  passed,  each  of  its  limbs  being  four  inches  long,  and  the  bore 


116 


NON-OXYGENATION  OF  PHOSPHORUS. 


being  ]th  of  an  inch  ;  the  outer  limb  was  fitted  with  a  scale.  After  having  opened  the 
cock,  a,  a  stick  of  dry  phosphorus,  e,  was  suspended  in  the  cylinder,  which  was  made 
very  clean  and  dry,  and  the  siphon  being  filled  with  water,  was  firmly  seated  in  its 
place  and  the  cock  closed.  Now,  as  the  phosphorus  oxydized,  the  metaphosphoric 
acid  was  removed  by  the  water  present,  and  the  level  falling  in  the  lateral  limb,  indi- 
cated what  quantity  of  oxygen  was  consumed,  and,  therefore,  the  rate  of  combustion 
of  the  phosphorus.  This  was  expected  to  give  a  more  accurate  estimate  of  any  chan- 
ges occurring  in  the  phenomenon,  and  was  accordingly  applied  to  detect  them. 

441.  Beams  of  light  of  different  colours  being  made  to  pass  at  different  times  through 
the  cylinder,  so  as  to  impinge  on  the  phosphorus,  attempts  were  made  to  ascertain  the 
rate  of  combustion  for  each,  as  also  for  the  white  light  of  the  sun,  and  in  the  shade. 
In  each  insulated  experiment,  the  fluid  in  the  gauge  sunk  with  great  regularity,  more 
rapidly  at  first,  and  then  more  slowly,  but  the  same  regularity  was  not  observed  in  dif- 
ferent trials.  At  one  time  the  phosphorus  would  consume  with  more  than  double  the 
rapidity  than  it  did  at  another,  though  to  all  appearance  under  identical  circumstances. 
If  the  slow  combustion  of  phosphorus  be  at  all  affected  by  the  action  of  solar  light,  it 
is  certainly  not  to  that  extent  which  Ritter  supposed.  So  far  from  extinguishing,  the 
violet  rays  do  not  exert  any  control  over  it,  or  if  they  do,  it  is  to  so  small  an  extent 
that  the  most  delicate  arrangements  fail  to  detect  it. 

442.  It  is  possible,  however,  that  atmospheric  temperature  may  exert  an  influence 
on  the  result.  During  the  trials  here  made,  a  thermometer  in  the  shade  ranged  from 
70^  Fall,  to  82°  Fah.  At  these  points  the  affinity  of  the  combustible  material  for  oxy- 
gen may  be  so  exalted  that  the  action  of  any  weaker  force  becomes  masked.  It  is  not 
stated  what  were  the  temperatures  at  which  the  alleged  results  were  obtained.  But  it 
is  most  probable  that  the  presence  of  extraneous  matter  was  the  cause  of  all  these  vari- 
ations. It  is  well  known  that  certain  compounds  of  hydrogen  and  carbon,  in  ex- 
tremely minute  quantity,  will  entirely  put  a  stop  to  the  oxydation  of  phosphorus ;  and 
during  the  course  of  these  trials,  I  have  had  abundant  reason  to  notice  errors  arising 
from  this  cause.  By  simply  wiping  out  the  cylinder  with  a  linen  cloth,  which  con- 
tained an  almost  imperceptible  trace  of  spirits  of  turpentine,  an  erroneous  result  like 
that  of  Ritter  was  at  once  obtained. 

443.  Decomposition  of  the  Salts  of  Silver. — Several  of  the  salts  of  silver  un- 
dergo a  remarkable  change  when  exposed  to  the  rays  of  light,  the  bromide,  the 
chloride,  and  the  nitrate  being  very  good  examples;  these,  which  are  all  white,  become 
of  a  dark  colour,  approaching  almost  to  black,  when  exposed  to  the  violet  rays ;  it  is 
stated  that  the  bromide  is  most  readily  affected,  yielding  a  brownish-black  colour. 

444.  If  a  piece  of  paper  be  soaked  in  a  solution  of  nitrate  of  silver,  and  then  dipped 
into  a  solution  of  bromide  of  potassium,  it  affords  a  very  advantageous  means  of  ma- 
king these  experiments.  The  chloride  may  occasionally  be  substituted  for  the  bromide 
of  silver. 

445.  A  beam  of  light  {fg.  65,  a  a)  entered  a  dark  chamber  horizontally,  and  was 
obstructed  in  its  course  by  a  plane  metallic  screen,  h,  having  a  hole  half  an  inch  in 
diameter  in  it.    The  beam,  c,  which  passed  through  this  aperture  fell  upon  a  flint 


PRISMATIC  SPECTRUM  ON  BROMIDE  OF  SILVER. 


117 


glass  equiangular  prism,  d,  and  was  dispersed  by  it,  the  spectrum,  e  f,  being  received  on 
the  table;  this  spectrum  was  about  three  inches  long.  And  now  a  piece  of  paper,  im- 
bued with  bromide  of  silver,  was  placed  to  receive  it,  with  the  intention  of  ascertaining 
how  far  the  discoloration  would  extend.  In  the  course  of  five  minutes  a  very  marked 
change  had  taken  place,  and,  on  examination,  it  was  found  that  the  deepest  tint  had 
been  occasioned  where  the  violet  blended  with  the  indigo  rays;  beyond  this,  even  in 
the  dark  space  beyond  the  spectrum,  tliere  was  a  stain,  as  also  as  far  in  the  spectrum 
as  where  the  green  light  merged  into  the  yellow,  an  effect  represented  in  fig.  66,  a  a, 
h  h,  being  the  spectrum ;  during  this  experiment  the  spectrum  was  kept  stationary. 
Again,  a  column  of  light,  three  inches  in  diameter,  converging  from  a  convex  lens  (a  a, 
Jig.  67),  was  intercepted  by  a  screen  of  pasteboard,  b  b,  which  had  a  circular  aperture 
in  it  half  an  inch  in  diameter ;  this  screen  was  placed  at  such  a  distance  from  the 
focus,  that  the  circular  section  of  the  cone  of  light  was  half  an  inch  in  diameter,  and, 
therefore,  passed  exactly  through  the  aperture  ;  a  piece  of  the  prepared  bromide  pa- 
per was  then  fastened  on  the  back  of  the  screen,  so  as  to  receive  the  condensed 
rays  which  passed  the  aperture.  In  a  few  moments  a  black  spot  appeared  about  the 
central  parts  of  the  paper,  and  at  the  end  of  the  experiment  there  was  an  intensely 
black  circle,  surrounded  by  a  brown,  ring-like  penumbra,  as  in^^.  68;  the  diameter  of 
the  black  spot  being  three  quarters  less  than  that  of  the  aperture  through  which  the 
light  passed. 

446.  Interference  of  the  Chemical  Ravs. — Under  certain  circumstances,  two 
aerial  vibrations,  each  of  which,  if  separately  striking  the  organs  of  hearing,  would  pro- 
duce a  musical  sound,  may  so  interfere  with  each  other  as  to  produce  an  unmelodious 
rattling,  or  even  silence.  Also,  two  rays  of  light,  whose  paths  bear  a  certain  relation 
to  one  another,  instead  of  increasing  each  other's  intensity,  may  have  a  directly  oppo- 
site effect,  and,  neutralizing  each  other,  produce  darkness.  It  becomes,  therefore,  a 
question,  not  only  of  mere  curiosity,  but  one  whose  bearings  are  important,  to  find  if 
the  chemical  rays  emitted  from  the  sun,  when  placed  under  similar  circumstances, 
exhibit  similar  phenomena.  For  then  analogy  would  lead  us  to  know  that  it  is  possible 
for  two  rays  of  heat  to  be  so  situated  with  regard  to  one  another,  as,  instead  of  ex- 
alting the  temperature  of  the  body  on  which  they  fell,  to  lower  it ;  or,  in  other  words, 
to  produce  actual  cold. 

447.  In  my  early  trials  for  the  solution  of  this  question  I  met  with  many  disappoint- 
ments, but  at  last  I  fell  upon  an  arrangement  which  yielded  positive  information.  It  is, 
however,  an  experiment  requiring  careful  manipulation.  A  horizontal  beam  of  light  being 
projected  into  a  room  by  the  apparatus  heretofore  so  often  referred  to,  at  the  extremity,  e  e 
{Jig.69'),  of  the  brass  tube,  a  double  convex  lens  of  short  focus  was  screwed;  this  brought 
the  rays  to  a  point  at  a  distance  of  three  quarters  of  an  inch  from  the  lens :  here 
they  were  obstructed  by  a  metallic  screen,  b  b,  having  a  round  hole,  c,  one  eighth  of  an 
inch  in  diameter,  perforated  in  it.  This  screen  revolved  about  a  vertical  axis  on  a  pil- 
lar, d,  so  that  it  could  be  brought  to  any  angle  with  the  incident  rays.  The  rays  passing 
through  the  round  hole,  c,  were  received  on  a  white  screen,  g  g,  at  a.  distance  of  six 
inches.    When  the  screen  b  b  received  the  incident  rays  perpendicularly  to  its  surface. 


118 


SALTS  DECOMPOSED  BY  LIGHT. 


then,  of  course,  the  image  thrown  on  the  screen  g  g  was  circular ;  but  if  the  screen  b  h 
was  made  to  receive  these  rays  at  an  acute  angle,  then  the  image  was  lenticular. 
Under  the  last  condition,  the  phenomenon  of  diffraction  is  represented  m  fig.  70,  where 
a  a  \s  the  screen,  h  h  the  lenticular  image  cast  on  it;  it  is  of  bright  white  light  except  at 
its  central  part,  c,  where  there  is  a  dark  image  produced  by  the  interference  of  the  pass- 
ing rays. 

448.  If,  in  such  an  arrangement,  the  chemical  rays  do  not  interfere  with  each  other 
so  as  to  neutralize  effects,  chemical  action  should  be  produced  in  every  part  of  the 
image,  even  including  its  central  part,  c ;  but  if,  on  the  other  hand,  these  rays  are  obe- 
dient to  the  same  laws  as  the  rays  of  light,  then,  in  the  central  parts  of  the  image,  no 
chemical  effects  should  ensue ;  the  problem  is,  therefore,  reduced  to  the  finding  how 
any  compound,  changeable  by  these  rays,  will  comport  itself  on  the  central  and  peripheral 
parts  of  such  an  image. 

448.  In  place  of  the  screen  jg"  ^,  a  substitute  was  used  consisting  of  two  thin  plates 
of  mica,  with  a  layer  of  bromide  of  silver  included  between  them  ;  these  were  mounted 
in  a  httle  ivory  frame,  abed  {fig.  71),  just  in  the  manner  that  objects  are  usually  mount- 
ed for  the  use  of  the  microscope,  and  the  lenticular  image  cast  upon  the  l)romide. 
After  an  exposure  of  five  minutes,  during  which  care  was  taken  to  keep  the  sun's  place 
perfectly  immovable,  and  also  to  avoid  all  local  tremour,  which  might  make  the  image 
traverse  on  the  bromide,  the  result  was  very  apparent,  being,  as  represented  in  fig.  72, 
of  the  natural  size  ;  the  peripheral  parts  being  of  a  deep  brown,  and  the  centre  yellowish 
white.  Viewed  through  a  lens,  the  boundary  line  was  not  sharp  and  distinct,  but 
seemed  to  merge  by  an  insensible  gradation  into  the  unaffected  part,  as  in  fig.  73.  The 
conclusion  to  be  drawn  from  this  result  possesses  no  common  interest ;  for  the  same 
reasoning  which  demonstrates  that  light  consists  of  undulations  of  an  clastic  medium, 
applies  here  also. 

450.  The  chemical  rays,  thus  closely  attending  the  luminous  rays,  and  being,  like 
them,  subject  to  the  forces  bringing  about  reflection,  refraction,  and  interference,  it 
would  become  a  matter  worthy  of  inquiry  to  find  whether  there  be  any  different  classes 
of  these  rays  analogous  to  the  different  coloured  rays  of  light,  or  the  unequally  refran- 
gible and  absorbable  rays  of  heat.  The  salts  of  silver  are  only  one  of  a  class  over  which 
the  chemical  rays  exert  an  action.  The  following  list  contains,  I  believe,  all  the  me- 
tallic salts  at  present  known,  in  the  constitution  of  which  changes  are  brought  abnnf 
by  exposure  to  the  sun: 

Chloride  of  manganese,  Iodide  of  mercury, 

Sulpho-cyanate  of  iron,  Chloride  of  mercury. 

Sulphate  of  nickel,  Bichloride  of  mercury, 

Carbonate  of  lead.  Chloride  of  silver. 

Carbonate  of  nickel,  Bromide  of  silver. 

Nitrate  of  bismuth,  Sulpho-cyanate  of  silver. 

Chloride  of  uranium,  Nitrate  of  silver. 

Sulphate  of  uranium,  Bromate  of  silver. 

Nitrate  of  uranium,  Chloride  of  gold. 

Chloride  of  copper.  Chloride  of  osmium  and  potassium. 


MOONLIGHT  AND  ARTIFICIAL  FLAMES  ARE  INACTIVE. 


119 


Besides  which,  there  are  two  others,  whose  constitution  is  not  well  known ;  one  pre- 
pared from  an  alcoholic  solution  of  the  double  chloride  of  platinum  and  sodium,  by 
the  action  of  chloride  of  potassium,  and  the  other  in  a  similar  manner  from  the  cyanide 
of  platinum. 

451.  The  changes  which  these  bodies  experience  are  of  different  kinds:  some  be- 
come black  and  some  turn  white ;  some,  as  the  sulphate  of  nickel,  undergo  change  of 
crystalline  arrangement.  If  we  are  to  take  the  chloride  of  silver  as  a  type  of  those 
bodies  which  undergo  partial  reduction,  it  will  be  found  probable  that  the  change  im- 
pressed on  them  is  only  superficial,  as  analysis  will  show.  But  we  cannot  tell  with 
certainty  whether  a  perfect  reduction  of  some  of  these  compounds  takes  place,  or 
whether  it  is  a  sub-salt  of  a  dark-gray  colour  that  results.  By  taking  advantage  of 
the  property  which  chloride  of  silver  possesses  of  subsiding  very  slowly  from  neutral 
solutions,  so  as  to  make  them  assume  a  milky  consistency,  we  may  present  it  in  a  state 
extremely  favourable  to  the  action  of  the  solar  ray.  For  if  a  thick  mass  alone  be  ex- 
posed, the  central  parts  will  not  undergo  the  same  change  as  the  exterior,  being  shielded 
by  them  from  the  sun.  A  milky  solution  will,  after  an  exposure  for  a  certain  tiuie,  be- 
come quite  clear,  the  chloride  precipitating,  owing  to  the  liquid  becoming  acidulous. 
Mechanical  agitation  being  then  resorted  to,  to  expose  fresh  surfaces  of  the  precipitate 
very  frequently  during  a  period  of  eight  or  ten  days,  and  care  being  taken  to  suffer  no 
dust  or  other  impurity  to  enter  the  vessel,  it  will  be  found  that  the  powder  has  become 
of  a  reddish  gray,  interspersed  with  little  particles  of  unchanged  white  chloride  ;  these, 
from  their  superior  density,  will  have  precipitated  more  readily  than  the  gray  particles; 
washing  and  decantation  will  therefore  readily  effect  a  separation  of  them.  One  hun- 
dred grains  of  the  dark  chloride  thus  treated  yielded,  on  analysis,  79  3  of  metallic  silver; 
that  quantity  contains,  therefore,  20*7  of  chlorine ;  it  has  lost  by  exposure  5-3  grains  of 
chlorine  of  the  quantity  originally  contained  in  it. 

452.  Other  analyses,  of  the  same  sample,  furnished  results  not  widely  varying  from 
this,  but  such  is  not  the  case  with  analyses  of  different  samples;  these  give  sometimes 
more,  sometimes  less  chlorine ;  they  prove  that  the  chloride  of  silver,  as  daikened  by 
iio^ht,  is  not  a  definite  compound,  bat  rather  a  mechanical  mixture ;  that  the  change  of 
composition  is  chiefly  confined  to  the  surface,  and  does  not  affect  the  interior  of  the 
particles  to  any  extent;  it  is  true  that  microscopic  observation  shows  them  to  have 
a  uniform  consistency  and  colour,  but  of  course  reveals  nothing  of  their  internal  char- 
acter. An  error  is  frequently  made  by  writers  who  describe  the  changes  happening  in 
this  partial  reduction ;  it  is  not,  as  they  say,  hydro-chloric  acid  which  is  evolved  when 
the  chloride  is  under  water,  but  it  is  chlorine,  as  is  made  very  evident  by  the  strong, 
disagreeable  odour  of  that  gas  when  the  experiment  is  conducted  in  close  vessels. 

453.  In  addition  to  the  hst  given  above  of  substances  changed  by  the  chemical  rays, 
there  are  some  others  which  exhibit  their  action  in  a  very  marked  manner.  Chlorine 
and  hydrogen  unite  together  with  an  explosion  ;  carbon  and  chlorine  are  also  made 
thus  to  unite  in  producing  the  per-chloride  of  carbon  ;  all  kinds  of  vegetable  colours 
are  bleached  ;  hydriodide  of  carbon  and  chloro-carbonic  acid  are  always  made  by  the 
action  of  solar  radiant  matter. 


120 


PERIHELION  MOTIONS. 


454.  It  has  been  stated  by  some  chemists  that,  while  the  violet  extremity  of  the  solar 
spectrum  blackened  chloride  of  silver,  there  are  other  parts  of  it  which  would  bleach  the 
salt  so  blackened.  But  it  is  not  so,  for  neither  does  any  part  of  a  very  dispersed  spec- 
trum, nor  a  beam  which  has  passed  through  a  variety  of  absorbing  media,  exert  such  an 
action.  These  experiments  I  tried  repeatedly,  under  all  the  conditions  of  variation  of 
temperature  and  brilliancy  of  the  solar  rays,  but  no  observation  led  to  the  inference 
that  there  was  any  change  of  colour,  or  any  sign  of  an  approaching  change,  even  after 
the  lapse  of  a  whole  month.  Indeed,  it  would  seem  that  the  state  of  the  case  does  not 
justify  any  such  expectation ;  when  the  chemical  rays  have  disengaged  the  chlorine,  it 
is  gone,  and  lost  forever  to  the  silver,  being  scattered  abroad  in  the  atmosphere;  if,  there- 
fore, the  substance  ever  regains  a  white  colour,  chlorine  must  have  been  purposely  fur- 
nished from  other  sources,  or  the  white  substance  said  to  result  must  be  some  compound 
of  unknown  ingredients. 

455.  The  light  of  the  moon  is  a  remarkable  example  of  luminous  rays  existing  with- 
out either  calorific  or  chemical  rays  ;  the  most  delicate  thermometric  arrangements  have 
hitherto  failed  to  show  any  rise  of  temperature  in  the  moonshine.  A  piece  of  paper, 
imbued  with  chloride  of  silver,  may  also  be  exposed  to  the  rays  of  the  full  moon,  con- 
verging from  a  glass,  and  it  will  not  exhibit  any  change  ;  this  I  proved  by  placing  such  a 
paper  in  a  situation  where,  for  a  whole  night,  the  rays  of  the  moon  could  reach  it.  And 
the  same  observation  applies  to  terrestrial  flames.  In  none  of  these  has  the  existence 
of  the  chemical  rays  been  detected.  Chloride  of  silver,  after  being  exposed  for  eight 
hours  to  the  bright  flame  of  an  argand  lamp  converged  by  a  lens,  retained  its  whiteness. 
The  same  effect  was  witnessed  when  the  flame  of  alcohol  tinged  red  by  strontian  was 
employed,  or  the  yellow  flame  produced  by  chloride  of  sodium,  and  the  green  of  boracic 
acid  ;  in  these  cases  the  periods  of  exposure  did  not  exceed  half  an  hour. 

456.  Of  Perihelion  Motions. — Probably  the  most  remarkable  effect  exhibited  by  the 
solar  rays  is  the  motion  they  produce  in  media  endued  with  much  mobility.  For  many 
years  it  has  been  known  that  camphor  exposed  in  a  bottle  to  the  rays  of  the  sun,  formed 
a  crystallization  on  that  side  of  the  vessel  nearest  the  luminary;  but  the  action  is  so 
slow,  and  requires  such  a  length  of  time  for  its  completion,  that  no  successful  investiga- 
tion has  been  made  as  to  the  nature  of  the  forces  in  operation.  Some  philosophers 
have  assumed,  upon  insufficient  ground,  however,  that  the  crystallization  was  effected 
on  the  most  illuminated  side,  merely  because  it  was  the  coldest,  as  we  know  that  vapours 
are  always  deposited  on  that  part  of  a  surface  the  temperature  of  which  is  the  lowest. 

457.  About  three  years  ago,  I  published  a  series  of  observations  on  this  point.  Hav- 
ing found,  from  some  theoretical  considerations,  that  the  crystallization  of  camphor  took 
place  in  vacuo  with  a  rapidity  convenient  for  experimental  investigation,  I  was  led  to 
make  an  extended  inquiry  into  the  whole  matter. 

458.  The  sun's  rays  have  the  power  of  causing  vapours  to  pass  to  the  perihelion 
side  of  vessels  in  which  they  are  confined,  but,  as  it  would  appear,  not  at  all  seasons  of 
the  year.  For  example,  I  have  a  certain  glass  fitted  up  for  making  these  observations, 
and  in  this  vessel,  during  the  months  of  December,  January,  and  part  of  February,  1836- 
37,  a  deposite  was  uniformly  made  towards  the  sun ;  during  the  months  of  March,  April, 


PERIHELION  MOTIONS.  j[21 

and  part  of  May  next  following,  although  every  part  of  the  arrangement  remained  to  all 
appearance  the  same,  yet  the  camphor  was  deposited  on  the  side  farthest  from  the  sun. 
From  May  until  the  present  date,  the  deposite  is  again  towards  the  sun.  It  does  not 
appear  that  any  immediate  cause  can  be  assigned  for  this  way  wardness.  Does  it  exist 
in  the  sun's  lights  or  in  changes  affecting  the  earth's  atmosphere  I  or  in  imperceptible 
changes  in  the  instrument  with  which  the  observation  is  made?  As  respects  the  latter,  I 
think  a  negative  answer  may  be  given  without  any  hesitation;  but  beyond  a  mere  ex- 
pression of  the  fact  that  these  anomalous  circumstances  do  occasionally  occur,  I  would 
not  be  understood  to  speak  decisively ;  if  periodic  changes  like  this  do  occur,  which  is 
doubtful,  they  have  not  been  watched  for  a  sufficient  length  of  time,  nor  iiave  I  made 
sufficient  variations  in  my  trials  to  be  able  to  refer  them  to  any  distinct  cause.  A  large 
bottle  containing  camphor,  which  has  been  deposited  therein  for  more  than  a  year  under 
ordinary  atmospheric  pressures,  has  uniformly  showed  a  crystallization  towards  the  light. 

459.  For  making  these  experiments  properly,  it  is  necessary  to  possess  an  air-pump 
receiver  ground  so  true  as  to  be  able  to  maintain  a  vacuum  for  several  hours,  or  even 
days.  A  less  perfect  jar  may  be  made  to  answer  by  fastening  it  down  to  the  pump 
plate  with  cement ;  it  will,  however,  be  liable  to  leak  when  the  cement  becomes  warm  by 
exposure  to  the  sun.  For  many  of  these  trials  a  barometer  tube  is  sufficient.  Those 
who  are  provided  with  a  good  pump  and  jars,  accompanied  with  their  proper  transfer 
plates,  will  have  no  difficulty  whatever. 

460.  Upon  the  plate  of  the  pump,  or  one  of  the  transferers,  a  a  {Jig.  74),  place  some 
camphor  in  a  watch-glass,  c,  supported  by  a  stand ;  over  this  place  a  bell-jar,  and  ex- 
haust until  the  difference  of  level  of  the  siphon  gauge  amounts  to  half  an  inch  or  less — 
the  farther  the  rarefaction  is  pushed  the  better — then  remove  the  arrangement  into  the 
sunshine.  In  the  course  of  five  minutes,  if  the  atmosphere  be  clear  and  the  sun  bright, 
small  crystalline  specks  will  be  found  on  the  side  nearest  to  the  sun  ;  these  continually 
increase  in  size,  and  at  the  end  of  two  hours  many  l)eautiful  stellated  crystals,  from  one 
eighth  to  half  an  inch  in  diameter,  will  be  found  on  that  side,  but  on  the  other  parts  of 
the  glass  only  a  few  straggling  ones  here  and  there.  This  appearance  is  represented 
in  fig.  75;  sometimes  the  whole  side  next  the  sun  is  covered  with  a  deposite  of  camphor, 
the  other  side  containing  none  at  all. 

461.  Having  made  a  torricellian  vacuum,  in  a  tube  upward  of  33  inches  long  and 
five  eighths  wide,  pass  into  it  a  piece  of  camphor,  which  will  rise  into  the  void.  This 
arrangement,  like  the  former,  when  kept  in  the  dark  shows  no  crystallization,  even 
though  so  kept  for  more  than  four  months;  but  on  bringing  the  vacuum  into  a  beam  of 
the  sun,  crystallization  rapidly  goes  on,  and  at  the  end  of  a  quarter  of  an  hour  the  ap- 
pearance is  such  as  is  represented  in  fig.  76.  It  is  not  important  that  the  temperature 
of  the  sunbeam  or  of  the  atmosphere  should  be  high  ;  this  is  an  experiment  which 
will  succeed  at  temperatures  varying  from  120°  Fah.  to  60°Fah.,  and  probably  at  much 
lower  degrees,  for  it  is  readily  performed  in  the  depth  of  winter. 

462.  It  is  not  a  phenomenon  connected  with  the  process  of  crystallization.  Take 
a  jar  twelve  inches  high  and  four  in  diameter,  quite  clean  and  dry,  place  it  over  a  glass 
of  water,  h  {fig.  77),  and  expose  it  to  the  sunshine.    In  this  experiment  it  is  not  re- 

a 


122 


DEW  OF  WATER  AND  MERCURY.— IODINE  AND  GOLD,  ETC. 


quired  that  there  should  be  a  vacuum  within  the  jar.  In  the  course  of  an  hour  or  two, 
there  will  be  a  copious  dew  at  a,  and  on  farther  exposure  drops  of  water  will  trickle 
down  the  side  of  the  glass,  but  on  the  opposite  side  not  the  least  cloudiness  will  be  found. 

463.  Barometers,  hung  up  in  such  a  position  that  the  sun's  rays  can  have  access  to 
them,  exhibit  an  analogous  appearance,  the  side  nearest  the  light  being  studded  with 
metallic  globules. 

464.  In  any  of  these  experiments  iodine  may  be  substituted  for  camphor,  provided 
mercury  is  not  present,  nor  any  other  substance  on  which  this  body  acts ;  the  most 
advantageous  method  of  using  iodine  is  by  heating  it  in  a  suitable  vessel,  and  when 
the  vessel  is  quite  full  of  vapour,  presenting  it  to  the  sun's  ray;  deposition  goes  on,  on 
the  sunny  side,  as  the  condensation  lakes  place. 

465.  Nor  is  it  requisite,  in  obtaining  these  results,  that  the  material  should  be  either 
gaseous  or  vaporous.  The  rays  of  light  have  the  property,  as  was  found  by  Count 
RuMFORD,  of  decomposing  an  aqueous  solution  of  chloride  of  gold  ;  on  making  this  ex- 
periment in  a  test  tube  one  third  of  an  inch  in  diameter  (^^fig.  78),  small  spangles 
of  metallic  gold  will  be  seen,  by  reflected  light,  on  the  side  towards  the  sun,  h;  by 
transmitted  light  it  appears  of  a  pale  green  tint,  as  is  the  colour  of  gold  leaf.  Here  we 
find  that,  under  certain  circumstances,  solutions  will  deposite  metallic  matter,  in  obe- 
dience to  the  same  laws  which  cause  the  crystallization  of  camphor  and  the  deposite  of 
aqueous  dew. 

466.  A  few  pieces  of  camphor  were  laid  on  the  plate  of  an  air-pump,  and  a  circle 
of  glass  two  inches  in  diameter  (a,  Jig.  79)  was  supported  on  a  pedestal  in  the  midst 
of  them,  the  upper  part  of  the  glass  being  four  or  five  inches  above  the  pump  plate ;  it 
was  then  covered  with  a  jar,  and  exhaustion  performed.  On  exposure  to  the  sun  for 
a  suitable  length  of  time,  numerous  crystals  were  found  on  the  jar,  but  none  on  the 
circular  plate,  although  it  had  received  the  full  beams  of  that  luminary.  This  experi- 
ment was  made  with  a  view  of  determining  what  peculiar  condition  a  glass  surface  was 
placed  in  by  exposure  to  the  light;  for  experimental  purposes,  the  rounded  form  of  the 
glass  receivers  being  very  unsuitable.  It  was  not,  therefore,  w  ithout  surprise  I  observed 
that,  however  long  the  plate  was  continued  in  the  beams  of  light,  no  crystallization 
would  ensue.  A  flat  surface,  however,  being  essential  in  the  course  of  experiment  pur- 
sued, trials  were  repeatedly  made,  by  various  changes  in  the  arrangement,  to  cause  a 
deposition  of  camphor  upon  such  a  crown  glass  plate ;  but  though  in  five  days  I  could 
procure  starry  crystals  upon  the  bell  jar  of  more  than  half  an  inch  in  diameter,  in 
no  instance  was  a  solitary  one  found  on  the  glass  plate. 

467.  Two  circumstances  may  determine  the  precipitation  of  camphor  crystals  on  a 
surface :  1st.  Reduction  of  temperature  ;  2d.  Increase  of  pressure.  To  the  former  we 
cannot  look  for  an  explanation  in  the  case  before  us,  for  there  is  an  actual  increase  of 
temperature  in  every  part,  and  more  especially  on  that  side  of  the  vessel  which  is  next 
to  the  sun.  Why,  then,  does  this  condensation  take  place  on  the  hottest  surface,  the 
side  nearest  to  the  sun  I  we  cannot  admit  that  the  rays  of  heat  have  any  active  part 
in  bringing  about  the  phenomenon.  On  the  other  hand,  they  ought  rather  to  exert 
a  contrary  effect,  antagonizing  the  powers  that  solicit  the  camphor  crystals  to  form. 


CURRENT  ACTION.  123 

and  driving  them  to  the  coldest  surface.  We  are  therefore  reduced  to  the  supposition, 
that  when  the  hght  of  the  sun  impinges  on  a  surface  of  glass,  it  places  that  surface  in 
such  a  condition  that  it  exerts  a  pressure  on  the  adjacent  medium,  innnediatelj  follow- 
ed by  a  condensation  of  that  medium.  The  state  of  the  force  here  spoken  of  applies 
to  the  glass  surface  alone ;  it  is  not  an  action  between  the  solar  ray  and  the  forces  that 
produce  crystallization,  seeing  that  it  equally  takes  place  in  the  deposite  of  aqueous  or 
mercurial  dew,  and  even  of  solid  gold  from  a  solution  of  its  chloride.  In  other  words, 
if  a  ray  of  the  sun  be  incident  on  a  surface  of  glass,  it  develops  a  force  of  attraction  on 
that  surface. 

468.  A  gaseous  medium,  having  its  temperature  disturbed  at  any  point,  has  a  current 
determined  in  it.  In  a  chamber,  such  as  the  bell  of  an  air-pump,  this  current  circulates 
round  the  walls,  ascending  on  the  hot  and  descending  on  the  cool  side ;  it  might  be 
supposed  that  to  this  circumstance  was  due  tbe  fact  of  no  crystals  being  found  on  the 
plate  of  glass  (466).  The  condensation  cannot,  however,  be  attributed  to  this  cause ; 
for  if  so,  a  lamp,  or  any  other  source  of  heat,  would  be  equally  effectual;  it  will,  how- 
ever, be  hereafter  shown  that  artificial  flames  tend  to  remove  these  depositions  from 
the  side  nearest  to  them,  and  cause  them  to  be  accumulated  in  the  colder  regions. 

469.  Beneath  a  receiver  (a,  Jig.  80)  a  cubical  bottle,  h,  having  flat  sides,  was  placed, 
and  in  the  bottle  a  few  pieces  of  camphor;  the  mouth  of  the  bottle  was  about  half  an 
inch  in  diameter,  and  was  left  open,  the  pressure  of  the  atmosphere  being  reduced  to 
li  inches  of  mercury.  Temperature  of  the  ray  57°  Fah.  On  examination,  after  the 
lapse  of  one  hour  and  twenty-five  minutes,  no  crystals  whatever  could  be  found  on  the 
receiver,  and  but  a  few  sparsely  scattered  on  the  sides  of  the  cubical  vial.  Now  there 
can  be  no  doubt  that  the  whole  receiver  was  full  of  camplior  vapour,  and  it  does 
not  appear  that  any  reason  can  be  assigned  for  the  anomaly  of  its  non-crystallization. 

470.  Will  artificial  light  produce  analogous  results  \  To  ascertain  this,  I  took  a 
glass  globe  about  one  inch  and  a  half  in  diameter,  with  a  neck  four  inches  long,  fitted 
it  with  a  stopcock,  and  introduced  within  it  a  drop  of  water.  The  vapour  of  this 
water  exhibited  extreme  mobility ;  exposure  to  the  clouds  caused  its  immediate  deposi- 
tion. A  farther  advantage  was  gained  by  the  use  of  this  apparatus,  for  by  heating  the 
globe  uniformly,  until  all  the  moisture  on  its  surface  was  vaporized,  and  then  allowing 
it  to  cool,  the  particles  of  water  readily  obey  the  forces  that  solicit  them.  This  glass 
globe,  supported  vertically  on  an  appropriate  stand  (a,  Jig.  81),  was  placed  at  a  distance 
of  nine  or  ten  inches  from  a  brightly-burning  argand  lamp,  A ;  to  protect  it  from  acci- 
dental currents  of  air,  and  from  irregularities  of  radiation  from  other  sources,  the  whole 
arrangement  was  covered  by  a  bell,  h  c,  open  at  both  ends,  and  about  fifteen  inches  high. 
It  appeared,  at  first,  that  a  thin  dew  lined  the  inside  of  the  whole  globe,  instead  of  being 
confined  to  one  part;  but  after  a  certain  space  of  time,  the  heat  which  passed  from  the 
lamp  through  the  protecting  glass  disturbed  the  results,  the  dew  being  driven  to  the  cold- 
est parts.  To  get  rid  of  the  effects  of  this  heat,  at  a  distance  of  about  three  feet  from 
the  lamp  (A,  Jig.  82),  a  double  convex  glass  lens,  c,  2^  inches  in  diameter,  was  placed, 
which  brought  the  rays  to  a  focus  at  a  distance  of  five  or  six  feet,  where  stood  the  glass 
globe,  a,  covered  with  its  protecting  jar.    The  globe  had  been  previously  shghtly  warm.ed. 


124 


TEMPERATURE  OF  THE  SIDES  OF  A  JAR. 


SO  as  to  expel  all  the  dew  from  its  surface,  and  give  it  a  uniform  temperature  ;  in  sev- 
eral trials  it  was  found  that  there  were  no  evidences  that  the  bright  flame  of  an  argaiid 
lamp  exerted  any  force  soliciting  the  vapour  of  water  to  move  towards  one  part  of  the 
glass  rather  than  another. 

471.  I  took  the  arrangement  of  462,  and  shut  it  up  in  a  dark  closet,  having  pre- 
viously made  the  jar  perfectly  clean  and  dry ;  it  remained  there  for  several  days,  that 
it  might  be  found  whether 'those  little  irregularities  of  temperature  which  occur  in  such 
confined  chambers  would  cause  this  dew  to  pass  to  one  side  of  the  glass  rather  than 
another ;  it  did  not  appear  that  such  was  the  case,  for  the  glass  was  as  free  from 
moistui'e  when  taken  out  as  when  shut  up.  And  now,  this  arrangement  being  placed 
in  the  window,  where  the  sun  was  brightly  shining,  exhibited  on  its  perihelion  surface, 
in  the  course  of  three  and  a  half  minutes,  a  pearly  dew;  and  in  six  minutes  drops  of 
water  were  trickling  down  that  side. 

472.  But  it  is  not  essential  to  the  success  of  this  last  experiment  that  the  solar  ray 
itself  should  impinge  on  the  vessel.  The  temperature  in  the  shade  being  94°  Fah.,  I 
placed  the  receiver  with  its  cup  of  water  in  a  window  having  a  northern  exposure,  and 
found  that  the  dew  readily  made  its  appearance  on  that  side  which  was  towards  the 
light. 

473.  It  might  be  suggested  that  when  a  vessel  is  exposed  to  the  sun,  that  part  of 
the  glass  which  is  nearest  to  him  may  actually  be  the  coldest;  such  an  opinion,  it  is 
evident,  rests  on  no  sufficient  grounds.  A  jar,  a  g  {Jig.  83),  was  taken,  of  such  dimen- 
sions that  it  could  receive  the  differential  thermometer,  c  dh,  the  balls  of  which,  h  and  c, 
touched  the  opposite  sides,  and  in  the  dark  the  liquid  stood  at  zero,  but  on  bringing 
it  into  the  sunshine,  if  the  side  a  was  exposed,  then  the  ball  c  was  warmest,  and  if  the 
side  g,  then  the  ball  h  was  warmest,  as  was  indicated  by  the  motion  of  the  liquid. 
Hence  we  know,  that  in  all  cases  where  crystals  of  camphor,  dew  of  water,  (fee,  are 
deposited  on  the  side  next  the  sun,  they  are  so  deposited  in  opposition  to  an  energetic 
force  which  tends  to  remove  them. 

474.  Light  which  has  suffered  reflexion  at  certain  angles  seems  to  have  undergone 
a  remarkable  modification,  being  no  longer  able  to  put  the  glass  into  such  a  condition 
that  it  can  cause  motion  towards  the  sun.  It  is  not  to  be  inferred  that  any  connexion 
is  here  traced  between  this  disturbance  of  the  condition  of  light  and  the  change  im- 
pressed on  it  by  polarization.  A  beam  of  the  sun  falling  on  a  plate  of  glass,  and  being 
reflected  at  an  an^le  of  45",  may  be  intercepted  by  any  of  the  arrangements  of  sections 
460,  461,  as  by  the  barometer  tube.  It  will  be  found  that  the  crystallization  proceeds 
with  considerable  rapidity,  not,  however,  on  the  perihelion  side  of  the  vessel,  but  on 
the  opposite  side.  It  is  probable  that  this  result  is  not  dependant  on  the  polarization  of 
light,  inasmuch  as  it  takes  place  equally  well  at  all  the  angles,  less  and  greater  than  the 
niaxinuim  angle  of  polarization  of  glass.  Light,  even  that  of  the  sun,  having  once 
undergone  reflexion,  has  received  some  determinate  impress,  which  disables  it  entirely, 
from  causing  camphor  to  crystallize  on  the  perihelion  side  of  vessels. 

475.  Another  remarkable  phenomenon  is  exhibited  by  the  following  arrangement : 
Take  a  receiver,  a  {_fig.  84),  twelve  or  fifteen  inches  high,  and  three  or  four  in  diameter; 


ACTION  OF  A  SCREEN.  125 

place  it,  as  usual,  upon  the  transfer  plate,  with  its  proper  charge  of  camphor,  c.  Then 
cover  it  with  a  tin  cylinder,  ef,o{  sufficient  dimensions,  so  that  all  the  light  may  be 
shut  out  except  at  one  place,  g,  where  there  is  a  hole  half  or  three  quarters  of  an  inch 
in  diameter.  Under  favourable  circumstances,  as  a  serene  sky  and  bright  sun,  let  the 
arrangement  be  exposed,  that  a  column  of  light  may  pass  through  the  aperture  g  into 
the  glass;  it  may  or  it  may  not  finally  fall  on  the  camphor  at  c.  It  would,  of  course, 
be  expected  that  a  collection  of  crystals  would  form  on  the  inner  surface  of  the  glass 
corresponding  to  the  aperture  g.  But  on  trial  it  is  not  so ;  for  however  bright  the  sun 
may  shine,  or  however  favourable  other  circumstances  may  be,  not  a  solitary  crystal 
will  make  its  appearance  either  there  or  on  any  other  part  of  the  vessel,  provided  its 
temperature  has  been  pretty  uniform.  On  an  exceedingly  calm  and  serene  day  in 
July,  1835,  when  every  circumstance  seemed  propitious,  I  made  this  experiment,  and 
because  the  jar  that  I  was  using  was  not  ground  sufficiently  true  to  fit  the  transfer 
plate  accurately,  it  had  been  fixed  thereon  with  common  cap  cement,  and  on  exposure 
to  the  sun,  the  temperature  of  the  whole  arrangement  rose  so  high  that  the  cement 
was  in  almost  a  semifluid  condition  ;  it  was  one  of  those  days  when  the  eye  cannot  be- 
hold the  sky  or  look  on  the  ground  without  pain,  yet  not  one  crystal  could  be  made  to 
appear  opposite  to  the  aperture.  But  on  taking  off*  the  metallic  screen,  and  exposing 
the  jar,  in  a  little  more  than  a  minute  small  specks  were  observable  on  the  glass,  and  in 
a  quarter  of  an  hour  its  perihelion  side  was  densely  coated  with  crystals.  How  are  we 
to  explain  this  ?  Do  the  edges  of  the  aperture  g  impress  any  change  on  the  passing 
light  ?  Or  is  the  glass  surface  placed  in  such  a  condition  that  it  can  no  longer  produce 
the  deposite  of  crystals  \  We  shall  see  hereafter  that  there  are  circumstances  yet  more 
remarkable,  which  put  us  in  possession  of  an  explanation. 

476.  For  the  proper  understanding  of  the  rationale  of  these  experiments,  it  is  required 
to  know  whether  it  be  essential  that  the  solar  ray  should  impinge  on  the  camphor  or 
not,  or  whether  the  action  is  exerted  on  the  vapour  only.  A  tube  was  therefore  taken, 
of  suitable  dimensions ;  in  the  lower  part  of  it  a  fragment  of  camphor  was  deposited, 
and  screened  as  much  as  possible  from  the  rays  of  the  sun,  while  its  upper  part  was 
freely  exposed.  Crystals  formed  without  difficulty  at  a  distance  of  three  or  four  inches, 
or  even  a  foot,  from  the  camphor,  but  there  appeared  to  be  a  limit  beyond  which  they 
did  not  readily  pass.  A  tube  four  feet  six  inches  long  and  two  inches  in  diameter  being 
exhausted,  did  not  show  on  its  exposed  end  any  appearance  of  crystallization.  Near  the 
camphor  the  deposite  was  pretty  copious,  but  in  advancing  from  it  the  crystals  were  more 
sparsely  scattered,  until,  towards  the  upper  extremity,  none  could  be  seen.  Now  the 
maximum  quantity  of  vapour  that  can  exist  in  a  void,  or  among  other  gases,  provided 
the  mixture  be  in  equilibrio,  depends  on  the  lovvness  of  the  temperature  of  any  one 
part  of  the  vessel ;  and  hence,  a  long  tube,  one  of  the  extremities  of  which  is  kept  cold, 
does  not  exhibit  these  configurations  readily,  because  the  quantity  of  vapour  in  it  is 
small,  owing  to  the  coldness  of  one  part  of  the  void  space.  It  is  not  necessary,  there- 
fore, that  the  sun  should  shine  on  the  camphor,  the  effect  of  the  rays  taking  place  en- 
tirely on  the  vapour  filling  the  void. 

.  477.  There  is  a  singular  action  which  certain  bodies  exert  over  this  process.  Take 


126 


PROTECTING  ACTION  OF  A  RING. 


a  receiver,  able  to  maintain  a  vacaum  for  some  time,  and  liaving  cut  out  a  ring,  a  {Jig. 
85),  of  tin  foil,  an  incli  and  a  lialf  in  internal  diameter,  and  half  an  inch  wide,  paste  it 
upon  the  receiver,  as  at  a  {  fig.  86)  ;  moreover,  accommodate  the  receiver  with  its  cam- 
phor, as  usual,  and  having  exhausted,  expose  it  to  the  direct  rays  of  light,  so  that  the 
ring  a  shall  be  on  the  perihelion  side.  In  the  course  of  a  short  time  that  surface  will 
be  found  studded  in  various  directions  with  crystals,  as  is  to  be  expected  ;  but  it  will  be 
found  that  none  of  these  crystals  come  within  a  certain  distance  of  the  ring,  and  that 
not  one  is  to  be  seen  within  the  circle  circumscribed  by  it.  The  ring,  therefore,  ex- 
erts a  kind  of  protecting  action  on  the  glass,  hindering  the  deposition  of  crystals  within 
certain  limits  ;  such  a  result  is  depicted  in  Jig.  87. 

478.  This  action  of  a  ring,  formed  of  good  conducting  materials,  might  be  sup- 
posed to  arise  either  from  its  adding  something  to  the  surface  of  the  glass,  or  taking 
something  away  from  the  glass  with  which  it  is  in  contact.  Or  it  might  be  imputed  to 
some  change  impressed  on  the  ray  of  light.  Take,  therefore,  a  ring,  a  {Jig.  88),  and 
place  it  before  the  receiver,  h,  at  a  distance  of  half  an  inch,  the  ring  being  of  the  same 
dimensions  as  in  the  last  experiment ;  it  will  be  discovered  that,  although  the  ring  does 
not  touch  the  glass,  it  still  protects  it,  no  crystals  coming  within  a  certain  distance  of 
the  regions  overshadowed  by  the  metal ;  and  even  at  a  distance  from  the  line  of  shadow 
not  a  crystal  is  to  be  seen,  nor  any  visible  in  the  illuminated  centre. 

479.  Even  after  crystals  have  been  formed  on  the  surface  of  the  jar,  if  it  be  placed 
in  the  sunshine  with  a  ring  before  it,  as  in  the  foregoing  experiments,  the  ring  will  be 
found  not  only  to  exert  a  protection  on  the  glass,  hindering  any  farther  deposite,  but 
will  even  remove  the  crystals  that  are  there. 

480.  This  is,  indeed,  a  remarkable  circumstance  ;  a  part  of  the  perihelion  surface 
is  shaded  from  the  sun,  and  thereby  rendered  cooler,  yet  the  crystals  deposite  themselves 
on  the  hottest  surface,  and  avoid  that  where  it  is  cold.  We  see,  now,  how  it  happens 
that,  in  the  experiment  of  admitting  a  column  of  light  through  a  hole  in  a  screen,  no 
crystalline  deposite  was  effected  ;  the  protecting  agency  of  the  metal,  whatever  its  power 
might  be  due  to,  hindered  it. 

481.  To  give  the  particulars  of  one  of  these  experiments.  On  the  11th  of  July  I 
prepared  an  arrangement  such  as  the  foregoing:  the  thermometer  in  the  shade  was  at 
76°  Fall.,  and  in  the  sun  at  99°  Fah.,  distance  of  the  ring  from  the  jar  half  an  inch,  its 
internal  diameter  three  quarters,  and  width  half  an  inch.  After  proper  exposure  the  jar 
was  examined;  there  were  no  crystals  on  that  part  opposite  the  central  opening  of  the 
ring,  and  the  nearest  crystal  to  the  internal  border  was  six  tenths  of  an  inch  distant 
from  where  the  shadow  was  projected  on  the  glass. 

482.  Vapour  of  water  exhibits  similar  phenomena  ;  a  thin  piece  of  tin  foil  in  the  form 
of  a  cross,  a  ring,  or  any  other  shape,  effectually  prevents  the  deposite  of  water  near  it. 

483.  Instead  of  placing  the  ring  outside  of  the  glass,  now  let  it  be  placed  on  the 
inside,  as  at  a  {fig.  89),  so  that  it  may  be  within  one  eighth  of  an  inch  of  the  surface. 
When  the  crystals  have  fully  formed,  it  will  be  discovered  that  the  ring  has  exerted 
the  same  kind  of  protecting  agency  that  it  did  when  on  the  outside  of  the  glass. 

484.  Hitherto,  a  class  of  bodies  has  been  tried  as  protectors  which  are  without  ex- 


I 


IS  THE  SURFACE  ELECTRIFIED  1  (27 

ception  good  conductors  of  electricity,  such  as  the  metals.  Certain  indications  led  me 
to  make  trial  of  resinous  matters,  which  are  non-conductors  of  electricity.  Having 
made  the  region  about  a  {fig-  86)  of  the  air-pump  jar  very  warm,  over  a  spirit  lan)p, 
a  ring  of  rosin  was  spread  on  it,  about  the  same  size  as  the  ring  of  tin  foil  which  had 
been  formerly  there.  This  ring  of  rosin  was  transparent,  admitting  the  light  to  pass  it 
readily,  and  at  a  certain  distance  appeared  of  an  amber  colour.  Having  arranged  the 
jar  as  usual,  and  exposed  it  to  the  sun,  after  a  certain  length  of  time  well-marked  crys- 
tals were  deposited  on  the  perihelion  side,  on  which  the  rosin  was ;  these  crystals  not 
only  came  up  to  the  verge  of  the  rosin,  and  filled  also  the  inner  circle,  but  were  found 
on  the  rosin  itself. 

4S5.  Metallic  plates  of  various  shapes,  and  under  various  circumstances,  were  ex- 
posed with  a  view  of  causing  condensation  upon  them  ;  it  was  not  found  possible, 
however,  either  to  cause  the  formation  of  aqueous  dew  or  crystalline  deposite,  except 
when  their  temperature  was  below  that  of  the  medium  in  which  they  were  exposed. 

486.  At  this  stage  of  the  inquiry  it  becomes  important  to  know  whether,  along  with 
the  rays  of  light,  of  heat,  and  of  chemical  action,  there  are  not  also  rays  of  radiant 
electricity  emitted  by  the  sun.  Almost  all  operations  which  disturb  the  equilibrium  of 
light  and  heat,  disturb  too  that  of  electricity,  and  it  is  well  known  that  upon  this  fact 
Dr.  Harf,  founds  the  explanation  of  the  action  of  certain  voltaic  arrangements,  espe- 
cially the  calorimotor  ;  an  explanation  the  correctness  of  which  later  researches  make 
more  probable.  If  light,  heat,  and  electricity  are  set  in  motion  by  the  force  of  chemical 
action,  and  are  often  found  coexisting,  there  is  nothing  improbable  in  meeting  theni  to- 
gether in  the  case  before  us.  It  is  very  true  that,  as  yet,  we  have  not  met  with  any 
example  of  electricity  under  what  we  understand  as  a  radiant  form,  but  that  it  consists 
of  undulations  of  an  elastic  medium,  like  the  undulations  of  light  and  heat,  is  not  to  f)e 
doubted.  The  experiments  of  Nobili  give  proof  of  an  interference,  analogous  to  the 
interference  of  the  rays  of  light,  which  has  served  so  well  to  refer  the  motions  of  that 
fluid  to  the  undulations  of  an  elastic  medium  ;  the  analogies  of  light  and  heat  are  every- 
where kept  up,  and  we  look  with  confidence  that  they  will  be  extended  hereafter  to 
electricity. 

487.  The  tendency  of  the  experiments  here  communicated  is  to  show  that  certain 
substances,  conductors  of  electricity,  have  the  faculty  of  depriving  glass  of  that  power  by 
which  it  causes  the  condensation  of  vapours  upon  it  when  exposed  to  the  sun  ;  that 
deposition  will  not  take  place  on  metallic  surfaces,  but  that  certain  vitreous  and  resinous 
bodies  interfere  in  no  manner  with  the  process.  The  inference  appears  obvious,  that 
electricity,  brought  into  play  in  some  unusual  manner,  is  the  cause  of  the  phenomenon. 

488.  By  the  action  of  the  solar  ray  electricity  of  high  tension  can  be  developed.  A 
copper  electrical  condenser  was  taken,  the  plates  of  which  were  about  one  fortieth  of 
an  inch  apart,  and  six  inches  in  diameter;  there  was  nothing  more  in  their  construc- 
tion than  is  met  with  in  the  usual  arrangement.  Another  condenser  was  also  pro- 
vided, which  was  connected  with  a  gold-leaf  electrometer,  the  plates  being  one  inch  in 
diameter,  and  separated  from  each  other  by  a  very  thin  coat  of  gum-lac  varnish.  Trials 
were  repeatedly  made  to  discover  whether  the  apparatus  was  trustworthy.   It  is  a  com- 


128 


IS  THE  SURFACE  ELECTRIFIED  t 


mon  complaint  against  instruments  intended  to  indicate  low  charges  of  electricity,  that 
they  furnish  evidence  of  an  accumulation  when  none  has  been  communicated ;  it  is 
necessary,  therefore,  to  examine  an  instrument  to  be  certain  that  this  objection  cannot 
be  preferred  against  it.  Having  obtained  this  preliminary  evidence  in  a  satisfactory 
inanner,  and  having  decided  the  effectual  goodness  of  the  instruments  in  other  partic- 
ulars, the  following  trial  was  made.  The  six-inch  condenser  was  exposed  to  the  sun- 
beam for  one  hour,  on  a  clear,  bright  day  ;  the  charged  plate  was  then  parted  and  ap- 
plied to  the  one-inch  condenser ;  the  plates  of  this  being  parted,  a  small  but  perfectly 
distinct  electric  action  was  obtained.  This  experiment  is  not,  however,  devoid  of  sources 
of  error,  as  from  the  friction  occasioned  by  touching  the  plate  of  one  condenser  with  the 
plate  of  the  other,  or  the  heating  action  of  the  ray,  which  might  cause  currents  of  air 
to  brush  over  it ;  but  it  was  ^"ound,  by  purposely  rubbing  one  plate  of  the  condenser 
on  the  other,  that  no  charge  of  electricity  could  be  produced,  even  if  the  friction 
were  continued  during  some  time  ;  and  on  maintaining  the  temperature  of  the  condenser 
at  the  same  point  to  which  it  was  brought  by  the  sunbeam,  in  order  to  produce  like 
currents  of  air,  no  divergence  whatever  of  the  gold  leaves  was  produced. 

489.  When  the  tension  of  electricity  is  high,  one  of  the  most  delicate  methods  of 
detecting  its  presence  is  by  the  light  it  emits  in  vacuo ;  the  excitation  caused  by  the 
tremulous  motion  of  a  colunni  of  mercury  in  a  barometer  tube  is  rendered  visible  by 
the  bright  light  it  gives  out,  when  no  other  method  could  discover  it.  On  this  princi- 
ple, attempts  were  made  to  detect  electrical  action  in  the  sunbeam,  by  exposing  metallic 
plates  of  large  dimensions  to  the  ray,  and  causing  any  electricity  they  might  gather  to 
give  out  light  in  the  vacuum ;  these  trials  did  not  prove  satisfactory. 

490.  It  has  been  stated  (439),  that  the  cloud  which  rises  from  phosphorus,  when 
slowly  oxidating,  is  endowed  with  great  mobility;  for  certain  purposes  it  makes  a  very 
good  electroscope.  When  a  piece  of  phosphorus  is  shielded  from  the  air  by  a  bell  jar, 
and  not  exposed  to  disturbing  action  of  any  kind,  a  fine  sheet  of  vapour  rises  vertically 
upward.  If,  at  a  distance  of  several  feet,  an  excited  stick  of  wax  be  presented,  the 
vapour  curls  from  its  path,  and  leans  over  to  the  side  of  the  glass  adjacent  to  the  cause 
of  the  disturbance.  If  such  a  jar  be  exposed  to  the  sun,  a  like  disturbance  is  exhib- 
ited :  as  soon  as  the  rays  fall  on  it,  it  seems  as  though  they  caused  each  particle  to 
repel  its  fellows;  the  straight  column,  which  before  passed  to  the  top  of  the  jar,  separ- 
ates into  confused  masses,  which  pass  forward  to  the  perihelion  side. 

491.  No  direct  proof  existing  that  rays  of  electricity  are  emitted  by  the  sun,  and  as 
it  does  not  fall  within  my  purpose  to  discuss  their  hypothetical  action,  it  may  be  suffi- 
cient to  give  the  proof,  that  if  the  surface  be  admitted  to  be  electrified,  these  deposites 
should  take  place.  If  a  receiver  be  taken,  and  on  any  part  of  its  interior  surface  a 
glass  rod  be  made  to  pass,  the  line  which  it  describes  will  be  stellated  with  camphor 
crystals,  on  exposure  to  the  sun  after  exhaustion.  This  curious  fact  was  first  observed 
in  the  case  of  an  exhausted  vessel,  which  had  a  small  siphon  gauge  shut  up  in  it,  the 
extremity  of  which  rested  against  the  glass  ;  by  accident  the  gauge  was  moved  half 
round  the  glass,  and  in  a  short  time  after  a  line  of  crystals  was  observed  coinciding 
with  the  line  of  motion  ;  it  was  found  possible  afterward  to  repeat  tliis  result  at  pleas- 
ure;  the  appearances  were  such  as  arc  represented  mjig.  90. 


/ 


ACTION  OF  ABSORBENT  MEDIA.  129 

492.  Upon  the  hypothesis  here  assumed,  the  deposite  of  crystals  becomes  a  pheDom- 
enon  analogous  to  the  curious  configurations  described  by  Lichtenburg,  when  powders 
are  dusted  on  the  surface  of  an  electrified  plate  ;  so  close  is  the  resemblance,  that  one 
who  sees  crystallization  produced  by  the  sun  for  the  first  time,  would  be  led,  almost 
involuntarily,  to  refer  both  to  the  same  cause;  suppose  it  granted,  that  when  light  falls  on 
any  surface,  that  surface  is  electrified,  it  will  exert  an  attraction  on  any  particle  in  its  vi- 
cinity ;  but  if  a  conducting  substance  be  placed  in  contact  with  the  surface,  not  only  will 
it  hinder  deposite  on  the  place  which  it  occupies,  but  also  it  will  rob  the  glass  around 
it  for  some  distance ;  here  we  find  an  explanation  of  the  action  of  a  tin  foil  ring. 
Again,  if  that  conducting  substance  be  so  placed  as  to  cast  its  shadow  on  the  glass,  no 
deposite  should  take  place  on  that  shadow,  nor  for  a  certain  distance  around  it,  because 
the  electricity  of  the  adjacent  parts  would  pass  towards  the  unelectrified  spaces,  thus 
conferring,  by  a  surface  conduction,  a  low  charge  to  all  the  shaded  parts. 

493.  We  meet,  however,  if  we  pass  beyond  these  simple  explanations,  with  so  many 
difficulties,  that  we  are  not  encouraged  to  seek  farther  confirmation  of  this  hypothesis ; 
there  are  some  facts  which  prove,  almost  demonstratively,  that  electricity  is  not  the  agent 
in  question.  If,  instead  of  a  ring  of  rosin,  we  make  use  of  a  ring  of  sealing-wax  or  a 
ring  of  pitch,  these,  though  they  are  non-condactors,  do  not  fail  to  protect;  the  action 
of  a  metallic  ring,  when  placed  inside  of  a  jar,  cannot,  so  far  as  I  know,  receive  any  ex- 
planation, especially  if  we  are  to  admit  the  non-conducting  power  of  a  space  filled  with 
camphor  vapour  only.  It  is  plain  and  obvious  that  transparency  and  opacity  have  no- 
thing to  do  with  it ;  glass  and  rosin,  it  is  true,  do  not  protect ;  but  oil,  which  is  equally 
transparent,  protects  as  powerfully  as  a  metal. 

494.  Are  we  to  refer  this  singular  action  to  the  rays  of  light,  to  the  rays  of  heat,  or 
to  the  chemical  rays  1  By  the  action  of  absorbent  media,  attempts  have  been  made  to 
determine  this  question.  A  barometer  tube  (/  de,fig.  91)  had  a  conical  tube  fixed  on 
its  outside,  so  that  the  interstice  could  contain  liquids  at  c  d,  without  leaking.  Into  this 
torricellian  vacuum  I  passed  a  piece  of  camphor,  and  exposed  the  arrangement  to  the 
sun ;  having  filled  the  interstice  with  water,  it  was  found  to  have  crystals  on  the  aphe- 
lion side,  there  being  a  ring  of  them,  as  at  e  e,  Jig.  92,  all  round  the  tube.  The  water 
was  then  poured  out,  and  a  solution  of  sulphate  of  copper  and  ammonia  introduced.  On 
examination,  it  was  found  that  on  the  side  nearest  the  sun  no  crystals  were  to  be  seen, 
but  on  the  other  side  there  was  a  dense  layer  of  them,  extending  exactly  half  way  round 
the  tube,  and  very  much  resembling  the  shape  of  Jig.  93.  A  yellow  liquid,  the  bichro- 
mate of  potassa,  was  next  introduced  :  a  result  to  all  appearance  exactly  like  the  former 
was  again  produced;  but  having  observed  that  the  thickness  of  the  media  had  a  very 
sensible  effect,  apparently  due  to  their  becoming  warm,  and  not  casting  off  their  caloric 
with  sufficient  rapidity  by  radiation,  I  made  an  alteration  in  the  arrangement,  by  inter- 
posing between  the  torricellian  vacuum  and  the  light  a  trough  capable  of  containing  the 
different  solutions.  This  trough  being  filled  with  solution  of  bichromate  of  potassa,  and 
the  ray  tested  that  it  could  not  blacken  chloride  of  silver,  in  about  one  hour  the  tube 
presented  the  following  appearance :  there  were  some  pretty  large  crystals  which  ex- 
tended round  the  tube,  as  at  a  Jig.  94,  which,  on  the  aphelion  side,  suddenly  mounted 

11 


130 


CAUSE  OF  THE  GREEN  COLOUR  OF  PLANTS. 


up,  froming  a  kind  of  hyperbola;  on  the  anterior  semi-circumference  not  a  solitary  one 
was  to  be  seen.  The  trough  being  now  filled  with  sulphate  of  copper  and  ammonia, 
the  arrangement  of  the  crystals  was  found  to  be  in  every  respect  like  the  former. 

495.  Supposing  that  this  result  might  in  some  measure  depend  on  the  ray  having 
been  subjected  to  reflexion  before  passing  through  the  trough,  I  repeated  the  trials  when 
the  sun's  altitude  was  small  enough  to  permit  the  rays  to  pass  without  requiring  reflex- 
ion, yet  still  the  same  results  were  uniformly  obtained ;  so  that,  whether  the  chemical 
or  the  calorific  rays  were  stopped,  crystallization  took  place  on  the  aphelion  side  of  the 
tube. 

496.  May  it  not,  therefore,  be  that  this  attractive  force  originates  whenever  the  cal- 
orific ray  impinges  on  a  surface  ?  It  does  not  necessarily  follow  from  the  phenomena 
that  any  peculiar  class  of  rays  is  emitted  by  the  sun,  which  bring  about  this  action  ;  but 
if  there  are  such,  it  is  a  question  of  interest  to  find  what  is  the  reason  that  good  con- 
ductors of  electricity  render  their  action  nugatory. 

497.  Botanical  authors  have  long  been  aware  of  the  important  eflects  which  solar 
radiations  exercise  over  the  colour  of  vegetables.  A  plant  which  grows  in  the  dark  is 
of  a  pale  wliitish  colour,  and  of  a  transparent  aspect,  possessing  none  of  that  greenness 
and  vigour  which  are  so  characteristically  developed  on  exposure  to  the  sun  ;  its  consist- 
ency is  watery,  and  although  its  growth  may  not  be  stunted,  its  appearance  is  very 
sickly,  its  secretory  actions  are  not  duly  performed,  and  all  its  vital  operations  are  car- 
ried on  in  a  depressed  way.  There  is  no  longer  any  evolution  of  nitrogen  from  the 
leaves,  and,  consequently,  no  apparent  production  of  oxygen  gas.  Light,  which  seems 
to  act  tnerely  as  a  stimulus  on  the  green  organs  of  vegetables,  indirectly  bringing  about 
the  decomposition  of  carbonic  acid,  though  accessory,is  not,  however,  essential  to  growth. 
In  subterranean  cavities,  and  places  far  removed  from  the  direct  solar  ray,  plants  have 
a  colour  of  their  own ;  and  in  the  abysses  of  the  ocean,  at  depths  to  which  no  solar 
beam  can  penetrate,  and  where  there  is  a  perpetual  night,  they  are  found  flourishing. 

498.  The  green  colour  of  leaves  is  presumed  to  be  an  immediate  consequence  of  the 
act  of  decomposing  carbonic  acid.  It  appears  to  me  that  there  is  some  obscurity,  if  not 
an  actual  error,  in  the  view  which  botanists  take  of  this  matter.  They  suppose  that, 
by  the  stimulus  of  light,  some  portion  of  the  green  matter  is  enabled  to  decompose  that 
gas  completely,  or  to  accomplish  its  actual  resolution  into  an  equivalent  volume  of  oxy- 
gen, with  the  entire  deposition  of  the  carbon  in  the  solid  form  ;  that  it  is,  moreover, 
this  carbon,  so  deposited,  that  gives  origin  to  the  green  colour,  seeing  it  forms  the  chro- 
mule  verte  itself.  Much  useless  ingenuity  has  been  thrown  away  by  some  chemists  in 
explaining  how  carbon,  the  colour  of  which  is  black  or  a  deep  blue,  can  produce  a  lively 
green;  and  even  if  their  supposition  that  the  modifying  action  of  a  yellow  tissue  spread 
over  it  were  correct,  of  which  there  is  much  doubt,  considering  the  thinness  of  that 
tissue  and  the  lightness  of  its  tint,  yet  certainly  we  have  no  necessity  to  resort  to  any 
such  explanation.  The  deposite  is  not  carbon  chemically;  it  contains  both  oxygen  and 
hydrogen  in  unknown  proportions.  Of  all  the  physical  characteristics  of  a  body,  col- 
our is  the  most  uncertain :  after  uniting  in  a  new  mode,  compounds  never  bear  the  col- 
ours of  their  constituents  ;  nay,  more,  carbon  itself  is  not  essentially  of  a  black  colour, 
as  the  diamond  proves. 


I 


PLANTS  GROW  IN  LjlGHTS  OF  VARIOUS  COLOURS. 

499.  To  a  deposite  of  some  compound,  in  which  carbon  enters  as  an  ingredient,  we 
are  to  refer  the  green  colour  of  leaves,  but  not  to  carbon  itself.  The  earlier  chemists, 
who  did  not  possess  those  extremely  delicate  methods  of  gas  analysis  which  are  now 
available,  misunderstood  this  matter.  They  stated  that,  on  exposing  a  plant  to  the  sun- 
shine, in  contact  with  carbonic  acid,  the  carbon  was  separated  in  a  concrete  state,  the 
oxygen  being  left,  but  such  is  not  the  fact ;  by  no  known  laws  can  such  a  change  be 
brought  about,  and  hence  any  reasoning  based  upon  it,  as  to  the  colour  of  plants,  is  ir- 
relevant. For  when  a  plant  exposed  to  the  sun  decomposes  carbonic  acid,  a  certain 
volume  of  oxygen  disappears  at  the  same  time ;  in  lieu  of  this,  and  in  obedience  to  the 
laws  which  guide  the  transit  of  gases  through  tissues,  an  equivalent  volume  of  nitrogen 
is  surrendered  by  the  plant  in  return.  Sometimes  it  is  carbonic  oxide  which  is  absorbed, 
sometimes  oxalic  acid,  or  other  compound  of  carbon  with  less  proportion  of  oxygen.  I 
do  not  here  indicate  from  whence  that  nitrogen  is  derived,  since  botanists  assert  that 
some  plants  contain  no  nitrogen  at  all;  it  may,  however,  exist  in  their  juices  as  gas  ex- 
ists in  spring  water,  or  may  be  retained  in  a  compressed  state  on  their  surfaces ;  it  is, 
however,  a  remarkable  fact  that  nitrogen  is  always  present. 

500.  The  carbon  thus  taken  from  the  acid  does  not  pass  through  the  tissue  of  the 
leaf  in  a  concrete  form,  or  give  rise  to  a  concrete  deposite;  it  bears  with  it  a  certain  part 
of  the  oxygen  with  which  it  was  formerly  united,  the  rest  being  set  free  ;  the  carbon 
and  oxygen  so  conveyed  into  the  plant,  entering  into  combination  with  hydrogen,  give 
rise  to  the  chromule  verte;  hence  we  see  that  the  green  colour  depends  indirectly  on 
the  decomposing  action;  that  when  this  goes  on  without  interruption,  that  is  fully  de- 
veloped. 

501.  I  took  five  pea  plants  out  of  the  garden,  as  nearly  resembhng  each  other  in  size 
and  other  particulars  as  might  be:  they  had  just  appeared  above  the  surface  of  the 
earth,  and  were  beginning  to  put  out  leaves.  These  plants  I  designate  by  the  numbers 
1,  2,  3,  4,  5.  Each  one  was  planted  in  a  small  glass  vessel  with  a  hole  in  the  bottom, 
for  the  purpose  of  supplying  it  with  water,  after  the  manner  of  a  common  flower-pot. 
Number  1  was  placed  in  a  box  into  which  light  passed  which  had  traversed  a  solution 
of  sulphate  of  copper  and  ammonia.  No.  2,  in  a  similar  box,  into  which  light  was  ad- 
mitted after  having  undergone  the  action  of  chromate  of  potassa.  No.  3  was  placed  in 
the  open  air.  No.  4,  in  a  box  into  which  light  had  passed  which  had  been  transmitted 
through  sulphocyanate  of  iron.  No.  5  was  shut  up  in  a  dark  closet.  This  arrange- 
ment was  completed  on  the  second  day  of  May.  With  a  pair  of  compasses  the  height 
of  each  plant  was  ascertained,  and  of  that  and  of  the  number  of  leaves  a  memorandum 
was  taken.    In  three  daj^s'  time  an  examination  was  made. 

No.  1  had  attained  three  times  its  former  height,  and  doubled  its  number  of  leaves. 
No.  2,  not  quite  twice  its  former  height,  no  new  leaves,  in  appearance  not  so  plump 
and  transparent  as  formerly. 

No.  3,  twice  its  former  size,  with  no  fresh  leaves. 

No.  4,  four  and  a  half  times  its  former  size,  and  double  its  number  of  leaves. 
No.  5,  three  and  a  half  times  its  former  size;  the  leaves  looked  yellowish. 

502.  It  is  here  proper  to  remark,  that  the  increase  of  size  is  not  to  be  taken  as  an 


132 


SEEDS  ALSO  GERMINATE  IN  RED,  YELLOW,  AND  BLUE  LIGHT. 


index  of  any  action  of  the  absorbing  medium.  Some  jears  ago,  I  had  occasion  to  no- 
tice that  rapidity  of  growth  was  greatly  influenced  by  the  quantity  of  aqueous  gas  in 
tlie  atmosphere.  Whether  the  observation  possesses  any  noveky,  I  am  not  prepared 
to  say ;  but  if  any  one  causes  plants  to  grow  in  glass  vessels  containing  the  maximum 
quantity  of  vapour  which  the  atmosphere  can  hold  at  the  temperatures  under  trial,  their 
unusual  increase  in  dimensions  will  present  a  strikingly  remarkable  phenomenon. 

^503.  In  fourteen  days  fi'om  the  commencement  of  this  experiment  another  examina- 
tion was  made. 

No.  1,  all  its  leaves  of  a  grass  green. 

No.  2,  of  a  darker  green. 

No.  3,  green,  but  of  a  bluish  tint  when  compared  vs^ith  a  plant  taken  from  the  garden. 
No.  4,  of  a  bright  green. 

No.  5,  pale  whitish  yellow,  with  no  fresh  leaves,  but  grown  to  thirteen  times  its  for- 
mer height,  and  apparently  in  a  vigorous  condition. 

With  respect  to  No.  4,  the  plant  under  sulphocyanate  of  iron,  I  was  not  aware,  at 
the  time  of  making  this  trial,  of  the  singular  properties  of  that  substance  in  relation 
to  light ;  in  the  course  of  a  fortnight,  which  had  elapsed,  the  solution,  from  being  of  a 
deep  blood  red,  had  become  perfectly  colourless.  No  reliance  is,  therefore,  to  be  placed 
on  this  result. 

504.  Among  a  number  of  experiments  which  were  instituted  with  an  intention  of 
illustrating  the  same  point,  and  which  gave  analogous  results,  it  may  be  mentioned  that 
the  seeds  of  common  garden  cress  were  caused  to  germinate  and  grow  in  the  boxes 
mentioned  above  ;  and  no  matter  what  was  the  substance  through  which  the  light 
passed,  the  young  plants,  after  reaching  a  certain  size,  were  always  green,  but  those 
which  grew  in  the  dark  had  yellow  leaves  and  white  stalks. 

505.  The  general  result  of  these  trials  goes  to  prove  that  it  is  not  this  or  that  species 
of  ray  which  gives  rise  to  the  colour  of  leaves ;  the  absence  of  the  chemical  ray,  or 
of  the  calorific  ray,  does  not  appear  to  affect  it,  nor  have  we  any  direct  proof  that  the 
calorific  ray  exercises  any  influence.  Humboldt  has  stated  that  in  the  mines  of  Ger- 
many, plants,  as  the  annua,  et  co7yip)-essa,  plantago  lanceolata,  &c.,  grow  in  recesses 
where  the  sun's  light  never  comes,  and,  provided  hydrogen  gas  be  present,  their  colour 
is  green.  In  the  Atlantic  Ocean  he  saw  a  marine  plant, y^CMS  vitifolius,  brought  up  from 
the  depth  of  190  French  feet,  where,  according  to  the  calculations  of  Bouguer,  the 
light  was  only  equal  to  that  emitted  from  a  candle  at  203  feet  distance,  and  yet  its 
colour  was  green.  Decandolle  mentions  that  artificial  light,  as  that  of  lamps,  gives 
the  same  result ;  a  proof  that  it  is  certainly  not  the  chemical,  and,  perhaps,  not  the 
calorific  rays,  which  cause  the  phenomenon. 

506.  Perhaps  light,  in  this  case,  acts  only  as  a  kind  of  stimulus  ;  it  would  be  desirable 
to  make  trial  of  some  j)lants  whose  leaves  are  naturally  white  ;  of  this  class  there  are 
several  individuals  ;  would  they  or  would  they  not  cause  the  decomposition  of  carbonic 
acid  !  From  many  indications,  it  is  not  improbable  that  there  is  a  variety  of  chemi- 
cal rays,  each  of  which  brings  about  changes  of  a  character  appropriate  to  itself.  As 
yet  we  have  not  learned  to  distinguish  these  from  each  other,  and  are  not  provided  with 


ACTION  OF  ABSORBENT  MEDIA  AND  IDEAL  COLORATION. 


133 


the  means  of  effecting  their  separation.  A  remarkable  observation,  which  appears  to  me 
to  be  very  much  in  point,  was  made  manj  years  ago  by  Professor  Silliman  ;  it  has  not 
obtained  that  attention  which  it  deserves  ;  he  states,  that  on  exposure  of  a  mixture  of 
chlorine  and  hydrogen  to  the  light  of  a  fire,  an  explosion  was  produced.  I  quote  the 
fact,  however,  only  from  memory,  and  have  endeavoured  to  substantiate  it  under  a 
variety  of  circumstances,  but  with  a  want  of  success  probably  due  to  the  absorbing  ac- 
tion of  the  glass  jars  used,  or  to  the  nature  of  the  light.  It  is  desirable  that  this  experi- 
ment should  be  once  more  repeated  ;  it  would  settle  an  important  point — that  chemical 
i-ays  of  different  characters  exist.  I  have  referred  to  this  before,  for  it  is  more  than 
probable  that  there  are  chemical  rays  not  absorbable  by  the  chromates  of  potassa. 


Note  added  to  the  foregoing  Chapter. 

{Being  a  Letter  to  the  Editors  of  the  London  and  Edinburgh  Philosophical  Magazine,  inserted  in  that  Journal  February,  1840.) 

AN  ACCOUNT  OF  SOME   EXPERIMENTS   MADE  IN  THE    SOUTH   OF   VIRGINIA  ON   THE  LIGHT 

OF   THE  SUN. 

507.  Gentlemen — I  have  just  seen  in  the  Journals  for  the  current  month  a  letter 
from  Sir  J.  Herschel  to  the  British  Association  for  the  Advancement  of  Science,  in 
reference  to  some  remarkable  actions  of  the  different  colours  of  the  solar  spectrum. 

508.  About  five  years  ago,  having  the  advantage  of  a  bright  and  almost  tropical  sky, 
I  amused  myself  with  attempting  a  repetition  of  Morichini's  experiment  for  the  magnet- 
izing of  steel,  and  was  led  to  some  results  in  respect  to  the  chemical  action  of  the 
sun's  rays,  which  appear  to  bear  very  much  on  the  subject  of  the  letter  above  alluded 
to.  Most  of  these  have  been  published  in  the  Journal  of  the  Franklin  Institute  of  Phil- 
adelpliia;  but  as  they  do  not  appear  to  have  been  noticed  in  England,  I  will  ask  the 
favour  of  a  page  or  two  of  your  excellent  Magazine,  to  give  my  testimony  on  a  subject 
which  now  appears  to  excite  so  nuich  interest. 

509.  If  you  pass  a  beam  of  the  sun's  light  through  a  solution  of  chromate  of  po- 
tassa, it  can  no  longer  blacken  a  piece  of  sensitive  paper ;  if  you  converge  the  light 
which  has  thus  passed  through  a  stratum  of  this  fluid,  by  means  of  a  lens,  chloride  of 
silver  will  remain  for  a  long  time,  without  much  change,  in  the  focus. 

510.  The  list  which  was  published  in  the  Journal  above  named  of  solutions  pos- 
sessing this  power,  is  as  follows : 

Bichromate  of  potassa.  Muriate  of  iron. 

Chromate  of  potassa.  Chloride  of  gold. 

Yellow  hydro-sulphuret  of  ammonia.        Chloride  of  platinum, 
Hydro-sulphuret  of  lime. 

511.  It  is  to  be  remarked,  that  every  one  of  these  solutions  is  yellow,  but  I  also  found 
that  a  great  many  vegetable  coloured  infusions  would,  in  like  manner,  absorb  the  chem- 
ical rays,  especially  those  which  have  a  yellow  tint. 

512.  When  I   exposed  pieces  of  paper  covered  with  a  layer  of  chloride  of  silver 


134 


PERIHELION  MOTIONS. 


to  a  beam  which  had  passed  through  the  red  sulpho-cyaoate  of  iron,  the  paper  became 
of  a  brick-red  colour ;  if  to  a  beam  which  had  passed  through  a  solution  of  sulphate  of 
copper  and  ammonia,  it  became  of  a  blue-brown;  and,  lastly,  on  exposing  a  piece  in  a 
box,  which  I  shall  presently  mention,  for  five  days,  to  light  which  had  been  acted  on  by 
bichromate  of  potassa,  it  became  perceptibly  of  a  faint  yellowish  green. 

513.  It  is  very  probable  that  there  exist  in  the  sunlight,  rays  having  particular  chem- 
ical powers. 

514.  A  beam  which  has  passed  through  bichromate  of  potassa  does  not  appear  to 
cause  the  union  of  a  mixture  of  chlorine  and  hydrogen.  I  kept  such  a  mixture  for 
several  hours  in  it,  and  could  not  perceive  any  change. 

515.  But  this  same  beam  can,  nevertheless,  enable  vegetable  leaves  to  effect  the  de- 
composition of  carbonic  acid.  I  took  a  wooden  box,  about  a  cubic  foot  in  dimensions, 
and  having  removed  its  bottom,  replaced  it  with  a  pair  of  parallel  plates  of  glass,  so  ad- 
justed that  there  was  an  interstice  between  them  of  half  an  inch,  or  thereabout.  Into 
the  trough  thus  formed  I  poured  a  solution  of  bichromate  of  potassa,  or  any  other  salt 
under  trial,  and  the  box  being  raised  on  one  end,  served  as  a  closet  in  which  bodies 
could  be  exposed  to  the  action  of  beams  that  had  passed  through  any  given  medium. 

516.  In  this  little  chamber,  its  trough  being  filled  with  a  solution  of  the  bichromate, 
I  placed  a  matrass  containing  water  slightly  impregnated  with  carbonic  acid,  and  a 
few  vegetable  leaves;  after  a  little  while,  air  bubbles  were  copiously  given  oft';  there 
had  been  placed,  similar  in  all  respects,  another  matrass  in  the  direct  rays  of  the  sun, 
and  when  a  quantity  of  gas  sufiicient  for  analysis  was  evolved,  it  was  found  that  car- 
bonic acid  had  in  both  cases  been  decomposed,  though,  as  might  have  been  expected, 
in  the  latter  more  energetically.  The  result  gave  a  mixture  of  carbonic  acid,  oxygen, 
and  nitrogen :  the  uniform  appearance  of  this  latter  body  was  subsequently  traced  to 
the  leaves  employed. 

517.  Plants  also  become  green  in  light  that  has  been  submitted  to  the  action  of  these 
yellow  salts,  and,  therefore,  deprived  of  the  rays  that  blacken  chloride  of  silver.  I  took  a 
number  of  pea  plants  out  of  the  garden,  in  May,  1837,  and  caused  them  to  vegetate  in 
light  modified  in  this  way,  and  also  in  light  which  had  passed  through  sulpho-cyanate 
of  iron,  and  sulphate  of  copper  and  ammonia,  &c.,  but  in  every  instance  the  leaves 
became  green.  It  may  also  be  mentioned  that  seeds  of  common  cress  were  caused  to 
germinate  and  grow  under  these  circumstances  ;  the  young  plants,  after  reaching  a  cer- 
tain size,  were  always  green,  but  those  which  had  grown  in  the  dark  had  yellow  leaves 
and  white  stalks. 

518.  Professor  Silliman  states,  in  one  of  the  early  numbers  of  his  Journal,  that  he 
witnessed  an  explosion  of  hydrogen  and  chlorine  caused  by  the  light  of  a  common  fire. 

519.  RiTTER  was  the  first  who  asserted  that  the  opposite  extremities  of  the  spectrum 
possess  opposite  powers  of  chemical  action ;  he  states  that  phosphorus  will  emit  fumes 
in  the  red  ray,  but  if  the  violet  be  thrown  on  it,  it  ceases  to  smoke ;  this  experiment  I 
repeated  often,  and  under  favourable  circumstances,  but  could  not  make  it  succeed. 

520.  I  could  succeed,  however,  in  showing  very  beautifully  the  interference  of  that 
class  of  chemical  rays  which  blacken  chloride  and  bromide  of  silver,  but  failed  in  trying 


THESE  PHENOMENA  PROBABLY  DUE  TO  RADIANT  HEAT.  235 

to  produce  their  polarization  for  want  of  proper  apparatus.  An  electric  current  circu- 
lating in  a  wire  does  not  seem  to  have  any  influence  on  these  chemical  rays ;  I  found 
that  the  same  neat  magnified  image  of  the  wire  was  obtained  on  chloride  paper  when 
it  was  placed  in  a  beam  diverging  from  a  lens,  whether  the  current  was  made  to  pass 
or  was  stopped. 

521.  So  much  for  chemical  actions :  let  me  now  ask  your  attention  to  a  mechanical 
result  of  solar  light,  which  is  very  curious. 

(a.)  Having  made  a  large  air-pump  jar  very  clean  and  dry,  place  a  few  pieces  of 
camphor  on  the  plate  of  the  pump,  and  exhaust.  Carry  the  pump  with  its  receiver  into 
the  sunshine,  and  very  soon  you  will  see  all  that  side  which  is  nearest  to  the  sun  cov- 
ered with  crystals,  but  there  will  be  few  or  none  on  the  side  which  is  farthest  from  him. 
With  the  brilliant  sun  of  Virginia  I  have  seen  this  effect  take  place,  and  beautiful  stel- 
lated crystals  appear  in  four  minutes,  literally  covering  the  whole  of  the  upper  parts 
of  the  jar  nearest  the  sun. 

(h.')  Or  make  a  tube  of  half  an  inch  or  more  in  diameter,  and  upward  of  thirty 
inches  long,  a  torricellian  vacuum ;  pass  up  through  the  mercury  a  fragment  of  cam- 
phor. The  tube  may  now  be  kept  for  any  length  of  time  in  the  dark  without  anything 
happening;  but  bring  it  into  the  beams  of  the  sun,  and  in  a  few  minutes  crystallization 
will  happen  on  the  side  next  the  luminary. 

(c.)  Again,  paste  on  the  inside  of  an  air-pump  jar  a  piece  of  tin  foil  an  inch  in  diam- 
eter, and  having  operated  as  in  experiment  (ci),  expose  this  side  towards  the  sun. 
Crystals  will  soon  form,  but  the  tin  foil  will  protect  the  glass  in  its  vicinity,  and  none 
will  be  found  within  a  certain  space  round  the  metallic  circle. 

(d)  CrystaUization  is  not  necessarily  connected  with  these  results :  the  vapour  of 
mercury  in  a  torricellian  void  is  condensed  towards  the  light ;  so,  also,  the  dew  which 
settles  on  the  inside  of  a  jar  containing  water  is  always  on  the  side  nearest  the  window. 
The  rays  of  the  sun  have  also  the  power  of  decomposing  a  solution  of  chloride  of  gold: 
the  metalline  spangles  are  deposited  on  that  side  of  the  glass  which  is  nearest  to  the  light. 

Artificial  light  gives  none  of  these  results. 

(e.)  Having  removed  the  piece  of  tin  foil  used  in  experiment  (c),  place  it  on  a  little 
stand  in  front  of  the  receiver;  it  will  hinder  the  crystallization  taking  place  in  the 
parts  on  which  its  shadow  is  cast,  and  also  for  a  certain  space  in  the  vicinity. 

(y!)  Take  a  jar  that  has  already  been  coated  with  crystals,  place  the  tin  foil  liefore 
it,  and  it  will  remove  all  those  crystals  which  are  within  its  shadow. 

(^.)  Instead  of  using  a  piece  of  tin  foil,  as  in  experiment  (c),  make  the  receiver  hot, 
and  rub  upon  it  a  piece  of  resin,  so  as  to  leave  a  transparent  circle  of  that  substance ; 
expose  to  the  light,  and  it  will  be  found  that  the  resin  cannot  protect  the  glass. 

(A.)  If  along  the  inside  surface  of  a  vessel  about  to  be  exposed  to  the  sun  a  glass 
rod  be  rubbed,  rows  of  crystals  will  be  deposited  on  the  lines  which  were  described  by  the 
end  of  the  rod,  but  the  vessel  must  be  very  dry  for  this  experiment  to  succeed. 

522.  Now  can  we  explain  these  singular  results  on  any  other  known  principle  than 
this :  That  the  side  of  the  jar  nearest  the  sun  radiates  freely  the  heat  that  it  receives, 
back  again,  while  radiation  is  interfered  with  at  the  other  side ;  that,  in  point  of  fact, 
the  anterior  side  is  the  colder,  and  the  other  the  hotter  \ 


136 


HISTORICAL  NOTE. 


CHAPTER  XL 

ON   THE    PROCESS   OF   DAGUERREOTYPE,   AND   ITS   APPLICATION   TO   TAKING  PORTRAITS 

FROM   THE  LIFE. 

Historical  Note. — This  chapter  contains  the  first  pubhshed  description  of  the  process 
of  taking  Daguerreotype  portraits.  Of  late,  both  in  America  and  in  Europe,  this  art 
has  been  much  cultivated  and  improved ;  it  now  forms  a  branch  of  industrial  occupa- 
tion. That  it  was  possible  by  photogenic  processes,  such  as  the  Daguerreotype,  to  ob- 
tain likenesses  from  the  life,  was  first  announced  by  the  author  of  this  volume  in  a  note 
to  tlie  editors  of  the  Philosophical  Magazine,  dated  March  31st,  1840,  as  may  be  seen 
in  that  Journal,  June,  1840,  page  535.  The  first  Daguerreotype  portraits  to  which  al- 
lusion is  made  in  the  following  chapter  (523)  were  produced  in  1839,  almost  immedi- 
ately after  M.  Daguerre's  discovery  was  known  in  America. 

It  may  farther  be  remarked,  that  of  those  spectral  images  which  have  excited  so  much 
attention  of  late  in  Europe,  under  the  name  of  Moser's  images,  an  account  is  here 
given,  and  given  in  connexion  with  the  explanation  of  the  Daguerreotype  (527-544). 
A  successful  attempt  was  made  in  Germany,  in  1842,  to  appropriate  the  discovery  of 
these  singular  phenomena  by  the  natural  philosopher  whose  name  now  stands  in  con- 
nexion with  them. 

Of  these  two  incidents  in  the  science  of  photography,  some  account  may  be  seen  in 
the  Edinburgh  Review  for  January,  1843.  In  that  work,  the  discovery  of  the  art  of 
taking  Daguerreotype  portraits,  and  the  first  observations  on  spectral  images,  are  attribu- 
ted to  their  true  source,  the  author  of  this  book.* 

*  "  He  was  the  first,  we  believe,  who,  under  the  brilliant  summer  sun  of  New- York,  took  portraits  with  the  Daguerreotype. 
This  branch  of  photography  seems  not  to  have  been  regarded  as  a  possible  application  of  Daguerre's  invention,  and  no  notice 
is  taken  of  it  in  the  reports  made  to  the  legislative  bodies  of  France.  We  have  been  told  that  Daguerre  had  not,  at  that  period, 
taken  any  portraits  ;  and  when  we  consider  the  period  of  time,  twenty  or  twenty-five  minutes,  which  was  then  deemed  ne- 
cessary to  get  a  Daguerreotype  landscape,  we  do  not  wonder  at  the  observation  of  a  French  author,  who  describes  the  taking 
of  portraits  as  toujours  un  terrainunpeu  fabuleuxpour  le  Daguerreotype.  Daguerre,  however,  and  his  countryman,  M.  Claudet, 
have  nobly  earned  the  reputation  of  having  perfected  this  branch  of  the  art. 

"  It  has  been  long  known  that  if  we  write  upon  a  piece  of  glass  with  a  pencil  of  soapstone  or  agalmatolite,  the  written  let- 
ters, though  wholly  invisible,  may  be  read  by  simpiy  breathing  upon  the  glass  ;  and  this  even  though  the  surface  has  been 
well  cleaned  after  the  letters  had  been  written.  Dr.  Draper  observed  that  if  a  piece  of  metal,  a  shilling,  for  example,  or  even 
a  wafer,  is  laid  upon  a  cool  surface  of  glass  or  polished  metal,  and  the  glass  or  metal  breathed  upon,  then,  if  the  siiilling  is 
tossed  from  the  surface,  and  tiie  vapour  dried  up  spontaneously,  a  spectral  image  of  the  shilling  will  be  seen  by  breathing 
again  upon  the  sur.face,  the  vapour  depositing  itself  in  a  different  manner  upon  the  part  previously  protected  by  the  shilling. 
More  recently,  Professor  Draper  has  shown  that  this  spectral  image  could  be  revived  during  a  period  of  several  months  of 
the  cold  weather  in  the  winter  of  1840-41  ;  but  he  has  stated  that  he  cannot  find  the  reason  of  this  result,  though  he  regards 
it  as  analogous  to  the  deposition  of  mercurial  vapour  in  the  Daguerreotype.  We  have  often  repeated  this  interesting  exper- 
iment by  keeping  the  protecting  body,  the  shilling  or  wafer,  at  a  distance  from  the  glass  or  metallic  surface,  or  by  putting  it 
under  a  watch-glass  ;  and  we  found  that  the  result  was  always  the  same  (even  after  cleaning  the  surface  with  soft  leather), 
so  that  change  of  temperature,  or  any  pressure  upon  the  glass  surface,  were  excluded  as  causes  of  the  phenomena." — (Ex- 
tract from  the  Edinburgh  Review  for  January,  1843,  p.  339.) 


DAGUERREOTYPE  PORTRAITS  FROM  THE  LIFE  FIRST  TAKEN. 


(From  the  London  and  Edinburgh  Philosophical  Magazine  for  September,  1840.) 

Contents  :  Daguerreotype  Portraits  froin  the  Life  first  taken. — Spectral  Images. — 
Preservation  of  the  Sensitive  Plate  increases  the  Sensitiveness. — Modificatioiis  in  the 
Daguerreotijpe  Process. — Moonlight,  Artificial  Light,  and  Drummond's  Light,  are 
all  active. — Description  of  the  original  Process  of  taking  Portraits  from  the  Life. 

523.  Very  soon  after  M.  Daguerre's  remarkable  process  for  photogenic  drawing  was 
known  in  America,  I  made  attempts  to  accomplish  its  application  to  the  execution  of 
portraits  from  the  life.  M.  Arago  had  already  stated,  in  his  address  to  the  Chamber 
of  Deputies,  that  M.  Daguerre  expected,  by  a  shght  advance,  to  meet  with  success,  but 
as  yet  no  account  has  reached  us  of  that  object  being  attained. 

524.  More  than  one  hundred  instances  are  recorded  in  Berzei.ius's  chemistry,  in 
which  the  agency  of  light  brings  about  changes  in  bodies  ;  these  are  of  all  kinds :  for- 
mations of  new  compounds,  rearrangements  of  elements  already  in  union,  changes  of 
crystallographic  character,  decompositions,  and  mechanical  modifications. 

525.  The  process  of  the  Daguerreotype  is  to  expose  a  surface  of  pure  silver  to  the 
action  of  the  vapour  of  iodine,  so  as  to  give  rise  to  a  peculiar  iodide  of  silver,  which, 
under  certain  circumstances,  is  exceedingly  sensitive  to  light.  The  different  operations 
of  polishing,  washing  with  nitric  acid,  exposure  to  heat,  &c.,  are  only  to  offer  a  pure 
silver  surface ;  the  operation  of  hyposulphite  of  soda,  and  the  process,  which  I  shall 
presently  describe,  of  galvanization,  are  to  free  the  plate  from  its  sensitive  coating,  and 
in  nowise  affect  the  depth  of  the  shadows,  as  some  of  the  French  chemists  at  first 
supposed. 

526.  There  is  but  one  part  of  the  Daguerreotype  which  does  not  yield  to  theory  : 
on  one  point  alone  there  is  obscurity.  Why  does  the  vapour  of  mercury  condense  in 
a  white  form  on  those  portions  of  the  film  of  iodide  which  have  been  exposed  to  the 
influence  of  light  \  condense  to  an  amount  which  is  rigidly  proportional  to  the  quantity 
of  incident  light  ? 

527.  Even  on  this  point  there  are  facts  which  appear  to  have  a  bearing 

(«.)  It  has  long  been  known  that  if  a  piece  of  soapstone  or  agalmatolite  be  made  use 
of  as  a  pencil  to  write  with  on  glass,  though  the  letters  that  may  have  been  formed  are 
invisible,  and  though  the  surface  of  the  glass  may  subsequently  have  been  well  cleaned, 
yet  they  will  come  into  view  as  soon  as  the  glass  is  breathed  on. 

(&.)  I  have  often  noticed  that  if  a  piece  of  very  clear  and  cool  glass,  or,  what  is  bet- 
ter, a  cold  polished  metallic  reflector,  has  a  little  object,  such  as  a  piece  of  metal,  laid 
upon  it,  and  the  surface  be  breathed  over  once,  the  object  being  then  carefully  removed,  as 
often  as  you  breathe  again  on  the  surface,  a  spectral  image  of  it  may  be  seen,  and  this 
singular  phenomenon  may  be  exhibited  for  many  days  after  the  first  trial  was  made. 

(c.)  Again,  in  the  common  experiment  of  engraving  on  glass  by  hydrofluoric  acid, 
if  the  vapour  has  been  very  weak,  no  traces  will  be  perceived  on  the  glass  after  the 
wax  has  been  removed  ;  but,  on  breathing  over  it,  the  moisture  condenses  in  such  a 
way  as  to  bring  all  the  object  into  view. 

(d)  In  (491),  (521)  I  have  described  a  phenomenon  which  relates  to  the  crystalli- 

S 


138     PRESERVATION  OF  THE  SENSITIVE  PLATE  INCREASES  ITS  SENSITIVENESS. 


zation  of  camphor  on  surfaces  of  dry  glass,  on  which  invisible  traces  have  been  made 
by  the  pressure  of  a  glass  rod  ;  this  also  appears  to  belong  to  the  same  class  of  effects. 

528.  Sekzelius  (Traite,  vol.  ii.,  p.  186)  has  attempted  to  explain  (a)  and  (c)  on 
this  principle,  that  the  changed  and  unchanged  surfaces  radiate  heat  unequally.  There 
may  be  strong  doubts  with  some  as  to  the  correctness  of  this,  but  is  not  the  Daguerre- 
otype due  to  the  same  cause,  whatever  it  may  be  1 

529.  We  must  separate  carefully  the  chemical  changes  which  iodide  of  silver  under- 
goes in  the  sunbeam  from  the  mechanical  changes  which  happen  to  the  sensitive  film : 
iodide  of  silver  turns  black  in  the  solar  ray ;  the  w  hole  success  of  the  Daguerreotype 
artist  depends  on  his  checking  the  process  before  that  change  shall  have  supervened. 

530.  The  coating  of  iodine  is  not  im7nediately  necessary  to  the  production  of  im- 
ages by  the  mercmial  vapour.  The  condition  seems  to  be  traceable  to  the  metallic  « 
surface.  If  you  take  a  Daguerreotype,  clean  off  the  mercury,  polish  the  plate  thor- 
oughly with  rottenstone,  wash  it  with  nitric  acid,  and  bring  it  to  a  brilliant  surface,  yet, 
if  it  has  not  been  exposed  to  heat,  the  original  picture  will  reappear  on  exposure  to  the 
mercurial  vapour.    Is  not  this  a  result  of  the  same  kind  as  those  just  referred  to  ? 

531.  As  a  polishing  material  for  the  Daguerreotype  plate,  common  rottenstone  and 
oil  answer  very  well.  The  plate  having  been  planished  by  the  workman,  is  to  be  rub- 
bed down  to  a  good  sm-face,  and  as  high  a  polish  given  to  it  as  possible ;  it  is  to  be 
heated  and  washed  with  nitric  acid,  as  indicated  in  the  French  account,  and  finished 
by  being  rubbed  with  whiting  {creta  praparata),  in  the  state  of  a  very  dry  powder,  go- 
ing over  it  for  the  last  time  with  a  piece  of  clean  dry  cotton ;  this  gives  an  intensely 
black  lustre,  which  cannot  be  obtained  by  rottenstone  alone,  and  thoroughly  removes 
any  film  which  nitric  acid  may  have  left. 

532.  To  coat  with  iodine,  I  make  use  of  a  box  about  two  inches  deep,  in  the  bottom  of 
which  that  substance  in  coarse  flakes  is  deposited ;  no  cloth  intervenes,  but  the  silvered 
plate,  with  a  temporary  handle  attached  to  it,  is  brought  within  half  an  inch  of  the 
crystals,  and  it  becomes  perfectly  coated  in  the  course  of  from  one  to  three  minutes  ;  no 
metallic  strips  are  necessary  to  ensure  this  effect;  if  the  edges  and  corners  are  thor- 
oughly clean,  the  golden  hue  will  appear  uniformly. 

533.  M.  Daguerre  recommends  that  the  plate,  after  being  iodized,  shall  be  placed 
in  the  camera  without  loss  of  time.  The  longest  interval,  he  says,  ought  not  to  exceed 
an  hour.  "  Beyond  this  space  the  action  of  the  iodine  and  silver  no  longer  possesses 
the  requisite  photogenic  properties." 

534.  There  may  be  something  peculiar  in  the  preparation  of  the  plate  as  I  have  de- 
scribed it,  but  it  is  certain  that  this  observation  must  be  received  with  some  limitation. 
A  plate  which  has  been  iodized  does  not  appear  so  quickly  to  lose  its  sensitiveness. 
On  the  other  hand,  by  keeping  it  in  the  dark  for  twelve  or  twenty-four  hours,  its  sen- 
sitiveness is  often  remarkably  increased.  Other  advantages  also  accrue.  Those  who 
have  made  many  of  these  photogenic  experiments,  will  have  had  frequent  occasion  to 
remark  that  the  film  of  iodine  is  not  equally  sensitive  all  over;  that  there  are  spots  or 
cloudy  places  which  do  not  evolve  any  impression ;  and  often  the  whole  is  in  thai 
condition,  that  the  bright  parts  alone  come  out,  while  the  parts  that  are  in  shadow 


MODIFICATIONS  IN  THE  DAGUERREOTYPE.  139 

do  not  evolve  correspondingly,  nor  can  they  be  well  developed,  except  at  the  risk  of 
solarizing  the  picture.  Now  a  plate  that  has  been  kept  for  several  hours  is  by  no 
means  so  liable  to  these  effects :  I  do  not  pretend  to  give  any  reason  for  this,  but 
merely  mention  it  as  a  fact  of  considerable  iuiportance  to  the  travelling  Daguerreotyper; 
he  will  find  that  the  iodine  does  not  lose  its  sensitiveness  in  many  days. 

535.  In  a  paper  read  before  the  Royal  Society,  of  which  an  abstract  is  given  in  the 
April  number  of  this  Journal  for  the  present  year  (p.  333),  Sir  John  Herschel  states 
that  there  is  an  absolute  necessity  of  a  perfect  achromaticity  in  the  object-glass  of  a 
photographic  camera.  M.  Daguerre  appears  to  have  been  under  the  same  impression, 
and  recommends  in  his  published  account  such  an  object-glass. 

536.  All  the  rays  of  light,  with,  perhaps,  the  exception  of  the  yellow,  leave  an  im- 
pression on  the  iodide  of  silver.  The  less  refrangible  rays,  however,  act  much  more 
slowly  than  those  which  are  at  the  opposite  end  of  the  spectrum.  In  the  common 
kinds  of  glass,  the  most  energetic  action  takes  place  in  the  indigo,  or  on  the  boundary 
of  the  blue.  Now  the  retina  receives  an  impression  with  equal  facility  from  each  of 
the  different  rays,  the  yellow  light  acting  as  quickly  upon  it  as  the  red  or  the  blue. 
Vision  is  therefore  performed  independently  of  time,  the  eye  catching  all  the  colours 
of  the  spectrum  with  equal  facility  and  with  equal  speed.  But  it  is  not  so  with  these 
photogenic  preparations.  In  the  action  of  light  upon  them,  time  enters  as  an  element; 
the  blue  ray  may  have  effected  its  full  change,  while  the  red  is  yet  only  beginning 
slowly  to  act ;  and  the  red  may  have  completed  its  change  before  the  yellow  has  made 
any  sensible  impression.  On  these  principles,  it  is  plain  that  an  achromatic  object- 
glass  is  by  no  means  essential  for  the  production  of  fine  photographs  ;  for  if  the  plate  be 
withdrawn  at  a  certain  period,  when  tbe  rays  tbat  have  a  maximum  energy  have  just 
completed  their  action,  those  that  are  more  dispersed,  but  of  slower  effect,  will  not  have 
had  time  to  leave  any  stain.    We  work,  in  fact,  with  a  temporary  monochromatic  light. 

537.  Upon  these  principles  I  constructed  the  camera  which  I  am  in  the  habit  of  using, 
with  a  double  convex  non-achromatic  lens.  Some  of  the  finest  proofs  were  procured 
with  a  common  spectacle  lens,  of  fourteen  inches  focus,  arranged  at  the  end  of  a  cigar 
box  as  a  camera ;  a  lens  of  this  diameter  answers  very  well  for  plates  four  inches  by 
three,  reproducing  the  objects  with  the  most  admirable  finish  ;  copperplate  engravings 
being  represented  in  the  minutest  particulars,  and  the  marks  of  the  tool  becoming  quite 
distinct  under  the  magnifier. 

538.  In  this  instance,  it  is  true,  owing  to  the  magnitude  of  the  focal  length  compared 
with  the  aperture,  but  little  difficulty  ensues  from  chromatic  aberration ;  but  when  with 
the  same  focal  length  the  aperture  is  increased  to  three  or  four  inches,  then  the  disper- 
sion becomes  very  sensible,  and  yet  good  proofs  can  be  procured  by  working  in  the 
method  here  indicated,  the  chief  difficulty  then  arising  from  spherical  aberration. 

539.  It  has  already  been  stated  that  the  ray  of  maximum  action  for  the  Daguerreo- 
type, when  colourless  French  plate-glass  is  used,  lies  probably  within  the  indigo  space; 
it  therefore  follows  that  the  length  of  the  camera  should  be  diminished,  after  arranging 
it  to  the  luminous  focus.  The  importance  of  this  is  pointed  out  in  a  paper  by  Mr. 
TowsoN,  inserted  in  this  Journal  last  year ;  I  was,  however,  in  the  habit  of  using  this 


140 


MODIFICATIONS  IN  THE  DAGUERREOTYPE. 


adjustment  before  reading  the  suggestions  contained  in  that  excellent  communication. 
The  amount  of  shortening  which  should  be  given  to  the  camera,  where  the  lens  is  fif- 
teen inches  focus,  does  not  commonly  exceed  three  tenths  of  an  inch.  If  the  luminous 
focus  be  used,  the  proof  comes  out  indistinct. 

540.  In  the  subsequent  process  of  mercurializing,  it  is  of  little  importance  what  is 
the  angular  position.  Several  experimenters  were  for  a  time  under  the  idea  that  an 
angle  of  45°  or  48°  was  a  necessary  inclination,  in  order  that  the  plate  should  take  the 
vapour;  this  arose  from  a  misinterpretation  of  the  printed  account.  Plates  mercurial- 
ize equally  well  in  a  horizontal  as  in  any  other  position  ;  perhaps  a  slight  inclination 
may  be  of  advantage,  in  allowing  the  vapour  to  flow  with  uniformity  over  the  iodized 
surface,  but  the  chief  use  of  an  angle  of  45°  is  to  allow  the  operator  to  inspect  the  pro- 
cess through  the  glass. 

541.  Sometimes  it  is  advantageous  to  heat  the  mercury  a  second  time,  when  the 
proof  is  not  distinctly  evolved  at  first.  Indeed,  it  occasionally  happens,  that  a  proof 
which  did  not  evolve  at  all  at  first,  will  come  out  quite  fairly  on  raising  the  tempera- 
ture of  the  mercury  again. 

542.  M.  Daguerre  recommends  two  methods  of  removing  the  sensitive  coating  from 
the  plate:  by  washes  of  hyposulphite  of  soda,  and  a  solution  of  common  salt.  The 
former  answers  perfectly,  the  second  only  indifferently  well.  There  is,  however,  another 
process,  which  is  very  simple,  and  has  an  advantage  over  the  former  of  these  in  cheap- 
ness. It  adds  not  a  little  to  the  magic  of  the  whole  operation,  in  the  eyes  of  those 
who  are  unaccustomed  to  chemical  results.  The  plate,  having  been  dipped  into  cold 
water,  is  placed  in  a  solution  of  common  salt,  of  moderate  strength ;  it  lies  without  be- 
ing acted  upon  at  all ;  but  if  it  be  now  touched  on  one  corner  with  a  piece  of  zinc 
which  has  been  scraped  bright,  the  yellow  coat  of  iodide  moves  off  like  a  wave  and 
disappears.  It  is  a  very  pretty  process.  The  zinc  and  silver  forming  together  a  vol- 
taic couple,  with  the  salt  water  intervening,  oxidation  of  the  zinc  takes  place,  and  the 
silver  surface  commences  to  evolve  hydrogen  gas;  while  this  is  in  a  nascent  condition 
it  decomposes  the  film  of  iodide  of  silver,  giving  rise  to  the  production  of  hydriodic  acid, 
which  is  very  soluble  in  water,  and  hence  instantly  removed. 

543.  This  process,  therefore,  differs  from  that  with  hyposulphite.  The  latter  acts 
by  dissolving  the  iodide  of  silver,  the  former  by  decomposing  it.  It  is  necessary  not  to 
leave  the  zinc  in  contact  too  long,  or  it  deposits  stains,  and  in  large  plates  the  contact 
should  be  made  at  the  four  corners  successively,  to  avoid  this  accident. 

544.  After  the  proof  is  washed,  all  the  defects  in  the  preparation  of  the  plate  become 
apparent.  If  a  film  of  mercury  has  existed  on  it,  due  to  its  not  having  been  burned 
sufficiently  long,  there  will  be  found  a  want  of  distinctness  in  the  shadows ;  or  if  the 
plate  has  not  been  burned  at  all,  perhaps  the  former  impressions  which  have  been 
obtaiiied  will  reappear.  This  accident  frequently  happened  in  my  earlier  trials,  when 
care  had  not  been  taken  to  give  a  due  exposure  each  time  to  the  spirit  flame.  Spectral 
appearances  of  former  objects,  on  different  parts  of  it,  emerged — an  interior  with  Paul 
Pry  coming  out,  when  the  camera  had  been  pointed  at  a  church. 

545.  There  is  no  difficulty  in  procuring  impressions  of  the  moon  by  the  Daguerreo- 


MOONLIGHT,  ARTIFICIAL  LIGHT,  AND  DRUMMOND'S  LIGHT,  ARE  ALL  ACTIVE.  J[4l 

type,  beyond  that  which  arises  from  her  motion.  By  the  aid  of  a  lens  and  a  heli- 
ostat,  I  caused  the  moonbeams  to  converge  on  a  plate,  the  lens  being  three  inches  in 
diameter.  In  lialf  an  hour  a  very  strong  impression  was  obtained.  With  another 
arrangement  of  lenses  I  obtained  a  stain  nearly  an  inch  in  diameter,  and  of  the  general 
figure  of  the  moon,  in  which  the  places  of  the  dark  spots  might  be  indistinctly  traced. 

546.  An  iodized  plate,  being  exposed  for  fifteen  seconds  only  close  to  the  flame  of  a 
gas  light,  was  very  distinctly  stained;  in  one  minute  there  was  a  very  strong  impression. 

547.  On  receiving  the  image  of  a  gas  light,  which  was  eight  feet  distant,  in  the  ca- 
mera, for  half  an  hour,  a  good  representation  was  obtained. 

548.  The  flame  of  a  gas  lamp  was  arranged  within  a  magic  lantern,  and  a  portion 
of  the  image  of  a  grotesque  on  one  of  the  slides  received  on  a  plate ;  a  very  good 
representation  was  procured. 

549.  With  Drummond's  light,  and  the  rays  from  a  lime-pea  in  the  oxy-hydrogen 
blowpipe,  the  same  results  were  obtained. 

550.  In  the  first  experiments  which  I  made  for  obtaining  portraits  from  the  life, 
the  face  of  the  sitter  was  dusted  with  a  white  powder,  under  an  idea  that  otherwise 
no  impression  could  be  obtained.  A  very  few  trials  showed  the  error  of  this ;  for 
even  when  the  sun  was  only  dimly  shining,  there  was  no  difficulty  in  dehneating  the 
features. 

551.  When  the  sun,  the  sitter,  and  the  camera  are  situated  in  the  same  vertical 
plane,  if  a  double  convex  non-achromatic  lens  of  four  inches  diameter  and  fourteen 
inches  focus  be  employed,  perfect  miniatures  can  be  procured,  in  the  open  air,  in  a 
period  varying  with  the  character  of  the  light,  from  20  to  90  seconds.  The  dress,  also, 
is  admirably  given,  even  if  it  should  be  black ;  the  slight  differences  of  illumination  are 
sufficient  to  characterize  it,  as  well  as  to  show  each  button,  button  hole,  and  every 
fold. 

552.  Partly  owing  to  the  intensity  of  such  light,  which  cannot  be  endured  without 
a  distortion  of  the  features,  but  chiefly  owing  to  the  circumstance  that  the  rays  de- 
scend at  too  great  an  angle,  such  pictures  have  the  disadvantage  of  not  exhibiting  the 
eyes  with  distinctness,  the  shadow  from  the  eyebrows  and  forehead  encroaching  on  them. 

553.  To  procure  fine  proofs,  the  best  position  is  to  have  the  line  joining  the  head  of 
the  sitter  and  the  camera  so  arranged  as  to  make  an  angle  with  the  incident  rays  of 
less  than  ten  degrees,  so  that  all  the  space  beneath  the  eyebrows  shall  be  illuminated, 
and  a  slight  shadow  cast  from  the  nose.  This  involves  obviously  the  use  of  reflecting 
mirrors  to  direct  the  ray.  A  single  mirror  would  answer,  and  would  economize  time, 
but  in  practice  it  is  often  convenient  to  employ  two :  one  placed,  with  a  suitable  mech- 
anism, to  direct  the  rays  in  vertical  lines  ;  and  the  second  above  it,  to  direct  them  in 
an  invariable  course  towards  the  sitter. 

554.  On  a  bright  day,  and  with  a  sensitive  plate,  portraits  can  be  obtained  in  the 
course  of  five  or  seven  minutes  in  the  diffused  daylight.  The  advantages,  however, 
which  might  be  supposed  to  accrue  from  the  features  being  more  composed,  and  of  a 
more  natural  aspect,  are  more  than  counterbalanced  by  the  difliculty  of  retaining  them 
so  long  in  one  constant  mode  of  expression. 


142 


ORIGINAL  PROCESvS 


555.  But  iu  the  reflected  sunshine,  the  eye  cannot  support  the  effulgence  of  the  rayg. 
It  is,  therefore,  absolutely  necessary  to  pass  them  through  some  blue  medium,  which 
shall  abstract  from  them  their  heat,  and  take  away  their  offensive  brilliancy.  I  have 
used  for  this  purpose  blue  glass,  and  also  ammoniaco-sulphate  of  copper,  contained  in  a 
large  trough  of  plate  glass,  the  interstice  being  about  an  inch  thick,  and  the  fluid  dilu- 
ted to  such  a  point  as  to  permit  the  eye  to  bear  the  light,  and  yet  to  intercept  no  more 
than  was  necessary.  It  is  not  requisite,  when  coloured  glass  is  employed,  to  make  use 
of  a  large  surface ;  for  if  the  camera  operation  be  carried  on  until  the  proof  almost  so- 
larizes, no  traces  can  be  seen  iu  the  portrait  of  its  edges  and  boundaries ;  but  if  the  pro- 
cess is  stopped  at  an  earlier  interval,  there  will  commonly  be  found  a  stain  correspond- 
ing to  the  figure  of  the  glass, 

556  The  camera  I  have  used,  though  much  better  ones  might  be  constructed,  has 
for  its  objective  two  double  convex  lenses,  the  united  focus  of  which  for  parallel  rays 
is  only  eight  inches ;  they  are  four  inches  in  diameter  in  the  clear,  and  are  mounted  in 
a  barrel,  in  front  of  which  the  aperture  is  narrowed  down  to  2>\  inches,  after  the  man- 
ner of  Daguerre's. 

557.  The  chair  in  which  the  sitter  is  placed  has  a  staff  at  its  back,  terminating  in 
an  iron  ring,  that  supports  the  head,  so  arranged  as  to  have  motion  in  directions  to  suit 
any  stature  and  any  attitude.  By  simply  resting  the  back  or  side  of  the  head  against 
this  ring,  it  may  be  kept  sufficiently  still  to  allow  the  minutest  marks  on  the  face  to  be 
copied.  The  hands  should  never  rest  on  the  chest,  for  the  motion  of  respiration  dis- 
turbs them  so  much  as  to  bring  them  out  of  a  thick  and  clumsy  appearance,  destroying 
also  the  representation  of  the  veins  on  the  back,  which,  if  they  are  held  motionless,  are 
copied  with  surprising  beauty. 

558.  It  has  already  been  stated  that  certain  pictorial  advantages  attend  an  arrange- 
ment in  which  the  light  is  thrown  upon  the  face  at  a  small  angle.  This  also  allows 
us  to  get  rid  entirely  of  the  shadow  from  the  background,  or  to  compose  it  more  grace- 
fully in  the  picture;  for  this,  it  is  well  that  the  chair  should  be  brought  forward  from 
the  background  from  three  to  six  feet. 

559.  Those  who  undertake  Daguerreotype  portraitures  will,  of  course,  arrange  the 
backgrounds  of  their  pictures  according  to  their  own  tastes.  When  one  that  is  quite 
uniform  is  desired,  a  blanket  or  a  cloth  of  a  drab  colour,  properly  suspended,  will  be 
found  to  answer  very  well.  Attention  must  be  paid  to  the  tint:  white,  reflecting  too 
much  light,  would  solarize  upon  the  proof  before  the  face  had  had  time  to  come  out,  and 
owing  to  its  reflecting  all  the  different  rays,  a  blur  or  irradiation  would  appear  on  all 
edges,  due  to  chromatic  aberration.  It  will  be  readily  understood  that  if  it  be  desired 
to  introduce  a  vase,  an  urn,  or  other  ornament,  it  must  not  be  arranged  against  the  back- 
ground, but  brought  forward  until  it  appears  perfectly  distinct  on  the  obscured  glass  of 
the  camera. 

560.  Different  parts  of  the  dress,  for  the  same  reason,  require  intervals,  differing  con- 
siderably, to  be  fairly  copied,  the  white  parts  of  a  costume  passing  on  to  solarization  be- 
fore the  yellow  or  black  parts  have  made  any  decisive  representation.  We  have,  there- 
fore, to  make  use  of  temporary  expedients.    A  person  dressed  in  a  black  coat,  and 


I 


FOR  TAKING  PORTRAITS.  143 

open  waistcoat  of  the  same  colour,  must  put  on  a  temporary  front  of  a  drab  or  flesh- 
colour,  or  by  the  time  that  his  face  and  the  fine  shadows  of  his  woollen  clothing  are 
evolved,  his  shirt  will  be  solarized,  and  be  blue,  or  even  black,  with  a  white  halo  around 
it.  Where,  however,  the  white  parts  of  the  dress  do  not  expose  much  surface,  or  ex- 
pose it  obliquely,  these  precautions  are  not  essential;  the  white  shirt  collar  will  scarce- 
ly solarize  until  the  face  is  passing  into  the  same  condition. 

561.  Precautions  of  the  same  kind  are  necessary  in  ladies'  dresses,  which  should  not 
be  selected  of  tints  contrasting  strongly. 

562.  It  will  now  be  readily  understood  that  the  whole  art  of  taking  Daguerreotype 
miniatures  consists  in  directing  an  almost  horizontal  beam  of  light,  through  a  blue-col- 
oured medium,  upon  the  face  of  the  sitter,  who  is  retained  in  an  unconstrained  posture 
by  an  appropriate  but  simple  mechanism,  at  such  a  distance  from  the  background,  or 
so  arranged  with  respect  to  the  camera,  that  his  shadow  shall  not  be  copied  as  a  part 
of  his  body  ;  the  aperture  of  the  camera  should  be  three  and  a  half  or  four  inches  at 
least ;  indeed,  the  larger  the  better,  if  the  object  glass  be  aplanatic. 

563.  If  two  mirrors  be  made  use  of,  the  time  actually  occupied  by  the  camera  oper- 
ation varies  from  forty  seconds  to  two  minutes,  according  to  the  intensity  of  the  light. 
If  only  one  mirror  is  employed,  the  time  is  about  one  fourth  shorter.  In  the  direct 
sunshine,  and  out  in  the  open  air,  the  time  varies  from  under  half  a  minute. 

564.  Looking-glasses  which  are  used  to  direct  the  solar  rays,  after  a  short  time 
undergo  a  serious  deterioration  ;  the  foil  assuming  a  dull  granular  aspect,  and  losing  its 
black  brilliancy.    Hence  the  time  in  copying  becomes  gradually  prolonged. 

565.  The  arrangement  of  the  camera,  above  indicated,  gives  reversed  pictures, 
the  right  and  left  sides  changing  places.  Mr.  Woolcott,  an  ingenious  mechanician 
of  this  city,  has  taken  out  a  patent  for  the  use  of  an  elliptical  mirror  for  portraiture  ; 
it. is  about  seven  inches  in  aperture,  and  allows  him  to  work  conveniently  with  plates 
two  inches  square.  The  concave  mirror  possesses  this  capital  advantage  over  the 
convex  lens,  that  the  proof  is  given  in  its  right  position,  that  is  to  say,  not  reversed ; 
but  it  has  the  serious  inconveniences  of  limiting  the  size  of  the  plate,  and  representing 
parts  that  are  at  all  distant  from  the  centre  in  a  very  confused  manner.  With  the  lens, 
plates  might  be  worked  a  foot  square,  or  even  larger. 

566.  Miniatures  procured  in  the  manner  here  laid  down  are  in  most  cases  striking 
likenesses,  though  not  in  all.  They  give,  of  course,  all  the  individual  peculiarities — a 
mole,  a  freckle,  a  wart.  Owing  to  the  circumstance  that  yellow  and  yellowish  browns 
are  long  before  they  impress  the  substance  of  the  Daguerreotype,  persons  whose  faces 
are  freckled  all  over  give  rise  to  the  most  ludicrous  results,  a  white  mottled  with  just  as 
many  black  dots  as  the  sitter  had  yellow  ones.  The  eye  appears  beautifully ;  the  iris 
with  sharpness,  and  the  white  dot  of  light  upon  it,  with  such  strength  and  so  much  of 
reality  and  life,  as  to  surprise  those  who  have  never  before  seen  it.  Many  are  per- 
suaded that  the  pencil  of  the  painter  has  been  secretly  employed  to  give  this  finishing 
touch. 


144 


OBJECT  OF  THE  MEMOIR. 


CHAPTER  XII. 

ON  SOME  ANALOGIES  BETWEEN  THE    PHF.NOMENA  OF  THE  CHEMICAL    RAYS    AND  THOSE 

OF   RADIANT  HEAT. 

(From  the  London,  Edinburgh,  and  Dublin  Philosophical  Magazine  for  September,  1841.) 

Contents  :  Object  of  the  Memoir. — On  the  Daguerreotyi)e  Process. — Chemical  Consti- 
tution of  Daguerreotype  Pictures. — Spectral  Images. — Film  of  Iodide  torn  off  Me- 
chanically.— Iodine  is  not  evolved,  hut  corrodes  the  Plate. —  The  Chemical  Rays  are 
absorbed. —  The  Photographic  Effects  are  transient. —  The  Chemical  Rays  are  not 
conducted.  —  They  become  Latent.  —  Optical  Qualities  control  Chemical  Action.  — 
The  Active  Rays  are  absorbed,  and  the  Complementary  reflected. — Relation  of  Op- 
tical Forces  and  Chemical  Affinities. 

567.  It  is  the  object  of  this  memoir  to  estabHsh  some  striking  analogies  which  ex- 
ist between  the  phenomena  of  the  chemical  rays  and  those  of  radiant  heat. 

568.  As  most  of  the  experimental  illustrations  which  I  shall  here  give  depend  upon 
the  use  of  M.  Daguerre's  preparation  (though  I  have  numerous  others  which  serve  to 
extend  these  truths  to  other  combinations,  and  which  will  be  published  in  due  time),  I 
shall  also,  incidentally,  give  what  appears  to  be  the  proper  theory  of  the  Daguerreotype, 

569.  Without  saying  anything  of  the  laws  of  reflexion,  refraction,  polarization,  and 
interference,  to  which  these  rays  are  subject,  the  study  of  which  I  commenced  more  than 
five  years  ago  on  paper  rendered  sensitive  by  the  bromide  of  silver,  farther  than  that  a 
general  similitude  holds  in  all  these  cases  between  the  rays  of  heat  and  the  chemical 
rays,  I  shall  at  present  confine  my  observations  to  establishing  the  following  propositions: 

1st,  That  the  chemical  action  produced  by  the  rays  of  light  depends  upon  the  ab- 
sorption of  those  rays  by  sensitive  bodies;  just  as  an  increase  of  temperature  is  pro- 
duced by  the  absorption  of  those  of  heat. 

2d.  That  as  a  body  warmed  by  the  rays  of  the  sun  gradually  loses  its  heat  by  radia- 
tion, or  conduction,  or  contact  with  other  bodies,  so  likewise,  by  some  unknown  pro- 
cess, photographic  effects  produced  on  sensitive  surfaces  are  only  transient,  and  gradu- 
ally disappear. 

3d.  That,  as  when  rays  of  heat  fall  on  a  mass  of  ice,  its  temperature  rises  degree 
by  degree,  until  it  reaches  32°  Fah.,  and  there  stops,  until  a  certain  molecular  change 
(liquefaction)  is  accomplished,  and  after  that  proceeds  to  rise  again,  so,  also,  the  chem- 
ical rays  impress  certain  changes  proportional  to  their  quantity,  up  to  a  certain  point, 
and  there  a  pause  ensues ;  a  very  large  amount  of  light  being  now  rendered  latent  or 
absorbed,  without  any  indication  thereof  being  given  by  the  sensitive  preparation  (as 
the  heat  of  fluidity  is  latent  to  the  thermometer),  a  molecular  change  then  setting  in, 
the  increments  of  the  quantity  of  light  are  again  indicated  by  changes  in  the  sensitive 
preparation. 


/ 

ON  THE  DAGUERREOTYPE  PROCESS.  145 

4th.  That  it  depends  on  the  chemical  nature  of  the  ponderable  material  what  rays 
shall  be  absorbed. 

5th.  That  while  the  specific  rays  thus  absorbed  depend  upon  the  chemical  nature  of 
the  body,  the  absolute  amount  is  regulated  by  its  optical  qualities,  such  as  depend  on 
the  condition  of  its  surfaces  and  interior  arrangement. 

6th.  It  will  be  proved  from  this,  that  the  sensitiveness  of  any  given  substance  de- 
pends on  its  chemical  nature  and  optical  qualities  conjointly,  and  that  it  is  possible  to 
exalt  or  diminish  the  sensitiveness  of  any  given  chemical  compound,  by  changing  the 
character  of  its  optical  relations.  We  shall  here  meet  with  an  explanation  of  some  of 
the  facts  noticed  by  Sir  J.  Herschel,  Mr.  Hunt,  Mr.  Talbot,  and  others,  respecting 
the  increase  of  sensitiveness  of  the  chloride  of  silver  and  other  bodies, 

7th.  That,  as  when  radiant  heat  falls  on  the  surface  of  an  opaque  body,  the  rays 
reflected  are  complementary  in  number  to  those  tliat  are  absorbed,  so,  in  the  case  of  a 
sensitive  preparation,  the  rays  reflected  are  complementary  in  number  to  those  that 
are  absorbed. 

570.  Of  the  Daguerreotypes. — In  relation  to  the  condition  of  these  tablets,  I  shall 
prove  the  following  facts  : 

1st.  That  metallic  mercury  exists  all  over  the  surface  of  an  ordinary  Daguerreotype, 
in  the  shadows  as  well  as  in  the  lights  ;  in  the  shadows  it  is  as  metallic  mercury,  in 
the  lights  as  silver  amalgam. 

2d.  That  in  an  iodized  Daguerreotype,  as  taken  from  the  mercury-bath,  there  is 
no  order  of  superposition  of  the  parts,  that  is  to  say,  the  iodide  is  neither  njjon  nor 
beneath  the  mercury,  but  both  are,  as  it  were,  in  the  same  plane. 

3d.  That  when  a  ray  of  light  falls  upon  the  surface  of  this  preparation,  through  all 
the  intervening  steps,  and  up  to  the  point  of  maximum  action,  no  iodine  is  evolved 
from  the  plate ;  but  that  in  the  common  Daguerreotype  the  light  comnmnicates  a  ten- 
dency to  the  atoms  of  the  iodide  to  yield  up  to  the  mercurial  vapour  their  silver,  while 
the  iodine  retires  and  combines  with  the  unaffected  silver  around.  It  follows  that  when 
such  a  plate  is  withdrawn  from  the  mercurial  vapour,  there  is  all  over  it  a  uniform  film 
of  iodide  of  silver  of  the  very  same  thickness  as  at  first ;  and  this  has  happened  through 
a  direct  corrosion  of  the  silver  by  the  iodine,  while  it  was  undergoing  the  mercurial 
operation. 

571.  I  pass  at  once  to  the  proofs  of  these  several  propositions,  commencing,  for  the 
sake  of  perspicuity,  with  those  relating  to  the  Daguerreotype  first :  and  1st,  That  me- 
tallic mercury  exists  all  over  the  surface  of  an  ordinary  Daguerreotype,  in  the  shadows 
as  ivell  as  in  the  lights ;  in  the  shadows  it  is  as  metallic  mercury,  in  the  lights  as  silver 
amalgam. 

572.  I  took  a  plated  copper  three  inches  by  four  in  surface,  and  having  prepared  it 
with  care,  I  exposed  half  of  it  to  the  diffused  light  of  the  day,  screening  the  other  half; 
it  was  then  mercurialized  at  175°  Fah.,  the  iodide  removed  by  hyposulphite  of  soda 
and  washed.  And  now  a  plate  on  which  a  gold  leaf  was  spread  was  placed  over  it, 
but  separated,  as  shown  in  Jig.  94,  in  the  points  a,  h,  c,  by  three  slips  of  glass.  By 
means  of  a  spirit-lamp  the  photographic  plate  a,  b,  c,  was  heated,  and  the  gilded  plate 

T 


146 


SPECTRAL  IMAGES. 


g  k  kept  cool,  by  occasionally  wetting  it.  On  parting  the  plates,  it  was  perceived  that 
faint  but  distinct  traces  of  whitening  were  visible  all  over  the  gold,  as  well  on  that  part 
which  was  over  the  whitened  half  of  the  photograph  as  over  that  which  was  unchanged. 

573.  But  as  it  might  happen  that  the  mercury  diffused  itself  laterally  past  the  imper- 
fect obstacle  b,  I  made  the  following  decisive  trials : 

I  iodized  three  silver  plates,  A,  B,  C,  each  three  inches  by  four  in  surface,  conducting 
the  processes  for  each  in  the  same  way ;  and  having  exposed  each  for  two  minutes  to 
a  faint  daylight,  I  laid  them  aside  in  the  dark,  to  be  presently  used  as  test  plates,  in 
lieu  of  the  gilded  plate  g  k. 

Then  I  took  three  other  plates,  D,  E,  F,  of  the  same  size,  and  conducting  the  pre- 
paratory processes  for  each  as  before,  I  iodized  D  in  the  dark,  and  mercurialized  it 
forthwith  at  170"  Fah.,  taking  the  utmost  care  that  not  a  ray  of  light  should  be  suffered 
to  impinge  upon  it. 

E  was  iodized,  and  exposed  for  two  minutes  to  diffused  daylight,  and  then  mercuri- 
alized at  170"  Fah. 

F  was  iodized,  and  exposed  to  the  sun  until  it  began  to  turn  brown,  an  effect  occur- 
ring almost  at  once.    It  was  then  mercurialized  at  170"  Fah. 

All  these  plates  then  had  their  sensitive  coating  removed  by  hyposulphite,  and  were 
thoroughly  washed  in  distilled  water  and  dried. 

574.  I  had,  therefore,  three  plates,  representing  accurately  the  conditions  proposed 
to  be  investigated.  D  was  in  the  condition  of  the  most  perfect  shadows,  E  in  that  of 
the  highest  lights,  and  F  solarized.  In  appearance  D  was  black,  E  was  white,  and  F 
bluish -gray. 

Upon  D,  E,  F,  I  placed  A,  B,  C,  respectively,  separating  each  pair  of  plates  one  six- 
teenth of  an  inch,  or  thereabout,  by  slips  of  glass.  Then  I  laid  them  on  the  level  sur- 
face of  the  sand-bath,  the  test  plates  being  kept  cool  by  sponging  occasionally  with  wa- 
ter.   Temperature  of  the  sand,  200"  Fah. ;  duration  of  the  experiment,  fifteen  minutes. 

On  examination.  A,  B,  C  were  all  found  powerfully  mercurialized,  nor  did  there  seem 
to  be  any  difference  between  them. 

575.  I  consider,  therefore,  that  the  shadows,  the  demitints,  the  lights,  and  the  solar- 
ized portions  of  a  Daguerreotype,  are  covered  with  mercury ;  for  at  a  temperature  of 
200°  Fah.,  they  all  evolve  it  alike,  a  sufficiency  of  vapour  rising  from  the  parts  that 
have  not  been  exposed  to  the  light  to  bring  a  plate  that  has  been  so  exposed  to  its 
maximum  of  whiteness.* 

576.  In  (527)  I  described  a  remarkable  effect  which  I  had  noticed  in  these  investi- 
gations: that  if  an  object,  such  as  a  wafer,  be  laid  upon  a  piece  of  cold  glass  or  metal, 
and  you  breathe  once  on  it,  and  as  soon  as  the  moisture  has  disappeared,  remove  the 
object,  and  breathe  again  on  the  glass,  a  spectral  image  of  the  wafer  will  make  its  ap- 
pearance. The  impression  thus  communicated  to  the  surface,  under  certain  condi- 
tions, remains  there  for  a  long  time.    During  the  cold  weather  last  winter,  I  produced 

»  I  believe  that  tlie  most  delicate  test  for  the  presence  of  mercury  is  a  slip  of  silver  iodized  to  a  yellow  colour,  and  expo- 
sed for  two  or  three  minutes  to  a  weak  daylight. 


FILM  OF  IODIDE  TORN  OFF  MECHANICALLY.  247 

such  an  image  on  the  mh-ror  of  my  heliostat ;  it  could  be  revived  by  breathing  on  the 
metal  many  weeks  afterward,  nor  did  it  finally  disappear  until  the  end  of  several  months. 

577.  I  do  not  at  present  know  what  is  the  reason  of  this  result,  but  the  analogy 
between  it  and  the  arrangement  of  mercurial  globules,  which  cover  the  surface  of  a 
Daguerreotype,  is  too  striking  to  be  overlooked.  It  proves  to  us  that  surfaces  may  as- 
sume such  a  condition  as  to  affect  the  deposition  of  vapours  upon  them,  so  as  to 
give  rise  to  the  reproduction  of  appearances  of  external  forms.  I  gave,  therefore, 
particular  attention  to  this  point,  but  eventually  found  that  silver  exists  in  an  ordinary 
Daguerreotype,  in  connexion  with  the  mercury,  all  over  the  plate,  in  a  less  proportion 
in  the  shadows,  and  in  a  greater  proportion  in  the  lights.  This  result  was,  however, 
only  obtained  after  the  following  fact  was  discovered  :  that  the  mucilage  of  gum-arabic, 
when  slowly  dried  in  a  thin  layer  on  the  surface  of  a  Daguerreotype,  splits  up  in 
shivers,  bringing  along  with  it  the  white  portions  of  the  picture,  and  leaving  the  plate 
clean. 

578.  Having,  therefore,  prepared  three  plates,  D,  E,  F,  exactly  as  before  (573),  I 
poured  on  them  a  solution  of  gum,  drained  them  so  as  to  leave  only  a  small  quantify, 
and  let  them  dry  slowly  over  the  sand-bath.  The  gum  separated  readily,  and  lay  in 
chips  on  the  surface  of  each  plate ;  it  was  easily  removed  to  three  sheets  of  paper, 
by  tapping  with  the  finger  on  the  back  of  the  plate.  Each  was  then  treated  alike,  as 
follows  : 

579.  The  gummy  matter  was  incinerated  on  a  platinum  leaf,  and  the  remaining 
ashes  transferred  to  a  test  tube,  half  an  inch  in  diameter.  One  drop  of  nitric  acid 
and  one  drop  of  water  were  added  ;  it  was  boiled  over  a  small  flame,  and  diluted 
with  a  little  water.  Dilute  muriatic  acid  was  now  added,  and  the  chloride  of  silver 
immediately  fell.  In  repeating  this,  it  is  necessary  to  attend  to  the  state  of  dilution  of 
the  acid,  for  if  too  strong,  it  wholly  dissolves  the  minute  quantity  of  chloride  of  silver 
generated. 

580.  As,  from  the  minuteness  of  that  quantity,  it  was  impossible  to  obtain  a  direct 
quantitative  analysis,  I  adopted  the  foregoing  method,  and  added  the  dilute  acid  to  all 
three  tubes  at  the  same  time.  In  D  there  was  a  faint  opalescence,  in  E  and  F  a  cloud  ; 
but  I  could  not  always  determine  whether  the  deposite  of  E  or  F  was  most  copious, 
sometimes  the  one  and  sometimes  the  other  appearing  to  have  a  slight  advantage. 

581.  I  conclude,  therefore,  that  while  the  whole  surface  of  the  plate  is  coated  with 
mercury,  it  exists  as  silver  amalgam  chiefly  in  the  lights,  and  as  uncombined  mercury 
chiefly  in  the  shadows,  and  in  a  mixed  proportion  in  the  demitints ;  and  that  when  a 
plate  is  solarized,  both  free  mercury  and  amalgam  are  present. 

582.  Such  is  the  state  of  surface  in  a  Daguerreotype  recently  formed.  In  the  course 
of  time,  however,  a  great  portion  of  the  mercury  that  is  in  the  shadows,  and  also  free 
in  the  lights,  evaporates  away.  When  the  picture  has  thus  changed,  the  shadows  are 
metallic  silver,  and  the  lights  silver  amalgam. 

583.  2d.  That  in  an  iodized  Daguerreotype,  as  taken  from  the  mercury -hath,  there 
is  vo  order  of  superposition  of  the  parts,  that  is  to  say,  the  iodide  is  neither  upon 
nor  beneath  the  mercury,  hut  hoth  are,  as  it  were,  in  the  same  plane. 


148 


IODINE  IS  NOT  EVOLVED,  BUT  CORRODES  THE  PLATE. 


Soon  after  I  had  ascertained  the  action  of  gum-arabic,  some  of  it  was  applied  to  the 
surface  of  a  plate  on  which  an  impression  had  just  been  formed  in  the  mercury- 
bath.  This  was  without  removing  the  coat  of  iodine.  On  drying  it,  the  gimi  chipped 
up,  as  was  expected,  bringing  away  with  it  all  the  lights  of  the  picture,  and  leaving 
a  uniform  coat  of  yellow  iodide  of  silver  beneath.  It  seems,  therefore,  that  the  film 
of  iodide  coheres  more  strongly  to  the  metal  plate  than  the  amalgam  ;  and,  farther, 
from  this  result  we  should  judge  that  the  amalgam  is  on  the  surface  of  the  iodide. 

584.  But  this  is  not  true ;  for  on  three  different  occasions  I  have  found  that  when 
Russian  isinglass  was  employed  instead  of  gum,  for  purposes  presently  to  be  related 
(591),  the  isinglass,  from  its  stronger  cohesive  power,  chipped  off  in  the  act  of  drying, 
tearing  up  the  yellow  film  from  end  to  end  of  the  plate,  and  leaving  the  amalgam, 
constituting  the  lights,  undisturbed.  It  is  here  to  be  understood  that  this  action  takes 
place  without  the  smallest  disturbance  of  the  lights  and  demitints,  the  plate  remain- 
ing in  all  the  beauty,  and  brilliancy,  and  perfection  that  it  would  have  had  if  it  had 
been  carefully  washed  in  hyposulphite  of  soda. 

585.  This  is  a  result,  however,  which  I  cannot  produce  with  uniformity.  Most 
conunonly,  the  lights  are  torn  up  with  the  iodide.  Had  it  occurred  but  once,  I  should 
still  have  cited  it  with  decision,  for,  from  the  very  character  of  it,  it  is  impossible 
to  be  mistaken,  or  to  commit  an  error  of  judgment.  It  proves  to  us  that  the  film 
of  iodide  may  be  mechanically  to7-n  ojfivom  the  metallic  surface  as  perfectly  as  it  can 
be  dissolved  offhy  chemical  agents — a  singular  fact. 

586.  This  result,  therefore,  proving  that  we  can  tear  off  the  film  of  iodide  and  leave 
the  amalgam,  can  only  be  co-ordinated  with  that  (583)  by  gum-water,  in  which  the 
amalgam  is  removed  and  the  iodide  left,  by  supposing  that  there  is  not  anything  like 
a  direct  superposition  in  the  case,  and  that  the  particles  of  amalgam  and  iodide  lie, 
as  it  were,  side  by  side. 

587.  3d.  That  when  a  ray  of  light  falls  upon  the  surface  of  this  preparation,  through 
all  the  intervening  steps,  and  up  to  the  point  of  maximum  action,  no  iodine  is  evolved 
from  the  plate,  hut  that  in  the  common  Daguerreotype  the  light  communicates  a  ten- 
dency to  the  atoms  of  the  iodide  to  yield  up  to  the  mercurial  vapour  their  silver,  while 
the  iodine  retires,  and  combines  with  the  unaffected  silver  around.  It  folloivs  that,  when 
such  a  plate  is  ivithdraivn  from  the  mercurial  vapour,  there  is  all  over  it  a  uniform 
film  of  iodide  of  silver,  of  the  very  same  thickness  as  at  first ;  and  this  has  happened 
through  a  direct  corrosion  of  the  silver  hy  the  iodine,  while  it  was  undergoing  the 
mercurial  operation. 

There  is  no  difficulty  in  proving  this  directly,  and  the  indirect  evidence  is  copious. 
If  we  lay  a  piece  of  paper  imbued  with  starch  on  an  iodized  plate,  and  expose  it  to 
the  sun,  although  the  plate  presently  assumes  a  dark  olive-green  colour,  the  starch  re- 
mains uucoloured. 

588.  This  dark  substance  is  probably  a  subiodide  of  silver ;  the  iodine,  therefore, 
which  has  been  disengaged  from  it  not  having  been  set  free,  must  have  necessarily  united 
with  the  adjacent  metallic  silver :  this,  for  very  obvious  reasons,  there  is  no  difficulty 
in  admitting. 


THE  CHEMICAL  RAYS  ARE  ABSORBED.  149 

589.  Now,  therefore,  when  a  photogenic  impression  existing  on  the  surface  of  a 
plate  in  an  invisible  state  is  brought  out  by  the  action  of  mercury  vapour,  we  easily 
understand  how  this  is  effected.  No  iodine  is  ever  evolved.  But  each  atom  of  iodide 
ot  silver  that  has  been  acted  on  by  the  light  yields  to  the  attraction  of  the  mercury 
its  atom  of  silver,  and  the  iodine  thus  set  free  unites  with  the  metallic  silver  particles 
around  it,  reproducing  the  same  yellow  iodide  by  a  direct  corrosion  of  the  plate :  the 
proofs  that  we  have  of  this  are  two  in  number. 

590.  1st.  Dry  some  mucilage  of  gum-arabic  on  a  Daguerreotype  just  brought  from 
the  mercury-bath  ;  when  it  has  split  up,  we  perceive  that  the  white  anialgam  of  silver 
is  removed,  and  a  uniform  coat  of  yellow  iodide  of  silver,  of  the  very  same  thickness 
as  at  first,  as  is  proved  by  its  colour,  is  left 

591.  2dly.  Dry  upon  the  same  plate  a  solution  of  Russian  isinglass,  and,  when  it 
has  split  up,  it  will  be  seen  that  it  uniformly  rends  off  with  it  the  yellow  iodide,  leaving 
the  metallic  plate  with  an  exquisite  polish  ;  aud  wherever  the  light  has  touched,  there 
it  is  corroded. 

592.  These  two  facts,  taken  together,  prove  that  in  mercurializing  a  plate  no  iodine 
is  evolved,  but  that  a  new  film  of  iodide  of  the  same  thickness  is  formed,  at  the 
expense  of  the  metallic  surface. 

593.  From  these  facts  we  readily  gather  that  on  the  presence  of  the  metallic  silver 
the  sensitiveness  of  this  preparation  mainly  depends,  for  to  the  tendency  which  the 
light  has  impressed  on  the  elements  of  the  iodide  to  separate  is  added  the  strong 
attraction  of  metallic  silver  for  nascent  iodine. 

594.  This  corrosion  or  biting  in  of  the  silver  plates,  by  the  conjoint  action  of  the 
mercury  and  iodine,  gives  rise  to  etchings  that  have  an  inexpressible  charm.  Could 
any  plan  be  hit  upon  of  forcing  the  iodine  to  continue  its  action,  the  problem  of  pro- 
ducing engraved  Daguerreotypes  would  be  solved.  By  another  process,  which  will 
be  described  hereafter,  I  have  succeeded  in  producing  deep  etchings  from  Daguerreo- 
types. 

595.  I  now  commence  with  the  proofs  of  the  leading  prooositions  set  out  with  in 
this  communication. 

And,  1st.  That  the  chemical  action  produced  hy  the  rays  of  light  depends  upon  the 
ABSORPTION  of  thosB  vays  hy  sensitive  bodies,  just  as  an  increase  of  temperature  is  pro- 
duced hy  the  absorption  of  those  of  heat. 

596.  Without  embarrassing  myself  here  with  any  considerations  of  the  tints  of  thin 
plates,  or  the  colours  of  natural  objects,  I  shall  use  the  term  absorption  as  expressive  of 
a  loss  of  radiant  matter,  whether  that  loss  arises  from  a  direct  union  of  the  luminous 
molecules  with  ponderable  matter,  or  is  rather  a  disappearance  of  effect,  caused  by  the 
interference  of  systems  of  undulations. 

597.  I  iodized  a  plate  to  a  golden-yellow  colour,  and  exposed  it  to  the  diffused 
light  of  day,  setting  it  in  such  a  position  that  it  reflected  specularly  the  light  falling 
upon  it  through  the  window  to  the  objective  of  a  camera-obscura,  which  formed  an 
image  of  it  upon  a  second  sensitive  plate.  The  beams  falling  upon  the  sensitive  plate 
of  course  exerted  their  usual  influence  upon  the  iodide,  which,  after  the  lapse  of  a  short 


150 


THE  PHOTOGRAPHIC  EFFECTS  ARE  TRANSIENT. 


time,  began  to  turn  brown.  As  soon  as  this  effect  was  observed,  I  closed  the  aperture 
of  the  camera,  and,  taking  out  its  plate,  mercurialized  it ;  but  it  was  found  that  the  rays 
reflected  from  the  sensitive  plate,  although  they  had  been  converged  by  a  lens  four 
inches  in  diameter,  and  formed  a  very  bright  image,  had  lost  the  quality  of  changing 
the  iodide  of  silver. 

598.  We  see,  therefore,  that  a  ray  of  light  which  has  impinged  on  the  surface  of 
yellow  iodide  of  silver,  has  lost  the  quality  of  causing  any  farther  change  on  a  second 
similar  plate  on  which  it  may  fall. 

599.  Iq  the  practice  of  photogenic  drawing,  this  observation  is  of  much  importance, 
especially  when  lenses  having  large  apertures  are  used;  the  rays  which  converge  upon 
the  sensitive  plate  are  reflected  by  it  in  all  directions,  and  the  camera  is  full  of  light ; 
its  sides  reflect  back  again  in  all  directions  on  the  surface  of  the  plate  these  rays,  which, 
if  they  were  effective,  must  stain  the  plate  in  the  shadows.  But  if  the  plate  has  been 
iodized  to  the  proper  tint,  this  light  is  wholly  without  action,  and  hence  the  proof  comes 
out  neat  and  clean. 

600.  Upon  an  iodized  plate  I  received  a  solar  spectrum  formed  by  a  flint-glass  prism, 
the  ray  being  kept  motionless  by  reflexion  from  a  heliostat,  and  the  plate  so  arranged  as 
to  receive  the  refracted  rays  perpendicularly.  After  five  minutes  it  was  mercurialized, 
and  the  resulting  proof  exhibited  the  place  of  the  more  refrangible  colours  in  the  most 
brilliant  hues.  The  lesser  refrangible  colours  had  also  left  their  impress  of  a  whitish  as- 
pect, but  the  region  of  the  yellow  was  unaltered.  All  the  different  rays,  therefore,  ex- 
cept the  yellow,  have  the  power  of  changing  this  particular  preparation.  Now  when 
a  number  of  pieces  of  cloth  of  different  colours  are  placed  in  the  sunbeam,  they  absorb 
heat  in  proportion  as  their  colour  is  deeper.  A  black  cloth,  which  does  not  reflect  any 
of  those  calorific  rays,  becomes  presently  hot;  and  in  the  same  way  Daguerre's  sensi- 
tive preparation  absorbs  all  the  rays  which  have  any  chemical  action  on  it,  and  reflects 
the  yellow  only,  which  does  not  affect  it.  In  this  particular  lies  the  secret  of  its  vast 
sensitiveness,  compared  with  the  common  preparations  of  the  chloride  and  bromide  of 
silver. 

601.  2d.  That  as  a  hodrj  warmed  hy  the  rays  of  the  sun  gradually  loses  its  heat  hy 
radiatio7i,  or  conduction,  or  contact  with  other  bodies,  so  likewise,  hy  some  unknown  pro- 
cess, photographic  effects  produced  on  sensitive  surfaces  are  only  transient,  and  gradually 
disappear. 

602.  After  a  beam  of  Ught  has  made  its  impression  on  the  iodide,  if  the  plate  be  laid 
aside  in  the  dark  before  mercurializing,  that  impression  decays  away  with  more  or  less 
rapidity ;  first  the  faint  lights  disappear,  then  those  that  are  stronger. 

603.  Having  brought  three  plates  to  the  same  condition  of  iodization,  and  received 
the  image  of  a  gas-flame  in  the  camera  on  each  for  three  minutes,  I  mercurialized  one, 
A,  forthwith  ;  the  second,  B,  I  kept  an  hour,  the  third,  C,  forty-eight  hours ;  the  rela- 
tive appearance  of  these  three  images  is  represented  in^^.  95. 

604.  Those  who  are  in  the  habit  of  taking  Daguerreotypes  know  how  much  they 
suffer  when  the  process  of  mercurialization  is  deferred.  To  show  this  effect  in  the 
extreme,  I  took  four  plates,  and  having  prepared  all  alike,  I  exposed  half  of  the  surface 
of  each  to  a  bright  sky  for  eight  seconds. 


THE  CHEMICAL  RAYS  ARE  NOT  CONDUCTED.  I5I 

No.  1,  mercurialized  immediately,        came  out  black  solarized. 

2,  "         in  five  hours,  "  white. 

3,  "         twenty-two  hours,       "      same  effect. 

4,  mercurialized  one  hundred  and  forty-four,  no  effect. 

605.  This  last  plate,  on  being  submitted  twice  more  to  the  vapour  of  mercury,  gave 
an  indistinct  mark.  On  exposing  a  corner  of  it  to  the  sun,  it  blackened  instantly  ; 
these  results  showing  that  the  peculiar  condition  brought  on  by  the  action  of  the  light 
gradually  disappears,  the  compound  all  the  time  retaining  its  sensitiveness. 

606.  Similar  results  are  mentioned  by  Daguerre  in  the  case  of  the  changes  produced 
on  surfaces  of  resinous  bodies,  and  I  have  noticed  them  in  a  variety  of  other  cases. 
Now  to  whatever  cause  these  phenomena  are  due,  whether  to  anything  analogous  to 
radiation,  conduction,  &c.,  it  is  most  active  during  the  first  moment  after  the  light  has 
exerted  its  agency,  but  it  must  also  take  effect  even  at  the  very  time  of  exposure ;  and 
it  is  for  these  reasons  that  it  comes  to  pass,  that  when  light  of  a  double  intensity  is 
thrown  upon  a  metallic  plate,  the  time  required  to  produce  a  given  effect  is  less  than 
one  half 

607.  I  could  conceive  the  intensity  of  a  ray  so  adjusted,  that  in  falling  upon  a  given 
sensitive  preparation,  the  loss  from  this  cause,  this  casting  off  of  the  active  agent,  should 
exactly  balance  the  primitive  effect,  and  hence  no  observable  change  result.  Hereafter 
we  shall  find  that  one  cause  of  the  non-sensitiveness  of  a  number  of  bodies  is  to  be  traced 
directly  to  the  circumstance  that  they  yield  up  these  rays  as  fast  as  they  receive  them. 

608.  It  needs  no  other  observation  than  a  critical  examination  of  the  sharp  lines  of  a 
Daguerreotype  proof  with  a  magnifying  glass,  to  show  that  the  influence  of  the  chemical 
rays  is  not  propagated  laterally  on  the  yellow  iodide  of  silver.  Of  the  manifestations 
which  these  rays  may  exhibit  after  they  have  lost  their  radiant  form  and  become  ab- 
sorbed, we  know  but  little.  If  they  conform  to  the  analogous  laws  for  heat,  and  if  the 
absorbing  action  of  bodies  for  this  agent  is  inversely  as  their  conducting  power,  we 
perceive  at  once  ivhy  a  photographic  effect  produced  on  yellow  iodide  of  silver  retains 
the  utmost  sharpness  without  any  lateral  spreading;  the  absorbing  power  is  almost  per- 
fect, the  conducting  should  therefore  be  zero. 

609.  2)d.  That,  as  when  rays  of  heat  fall  on  a  mass  of  ice,  its  temperature  rises  degree 
by  degree,  U7itil  it  reaches  32°  Fah.,  and  there  stops,  until  a  certain  molecular  change 
(liquefaction')  is  accomplished,  and  after  that  proceeds  to  rise  again,  so,  also,  the  chemical 
rays  i)npress  certain  changes  proportional  to  their  quantity,  up  to  a  certain  point,  and 
there  a  pause  ensues ;  a  very  large  amount  of  light  being  now  rendered  latent  or  absorbed, 
without  any  indication  thereof  being  given  by  the  sensitive  preparation  (as  the  heat  of 
fluidity  is  latent  to  the  thermometer'),  a  molecular  change  then  setting  in,  the  increments 
of  the  quantity  of  light  are  again  indicated  by  changes  in  the  sensitive  preparation. 

610.  Although  in  the  sun  the  iodide  of  silver  blackens  at  once,  this  is  only  the  result 
of  a  series  of  preliminary  operations. 

611.  When  we  look  at  a  Daguerreotype,  we  are  struck  with  the  remarkable  grada- 
tion of  tint,  and  we  naturally  infer  that  the  amount  of  whitening  induced  by  mercuriali- 
zation  is  in  direct  proportion  to  the  amount  of  incident  light ;  otherwise  it  would  hardly 
seem  that  the  gradation  of  tones  could  be  so  perfect 


152 


THE  CHEMICAL  RAYS  BECOME  LATENT. 


612.  But,  in  truth,  it  is  not  so.  When  the  rays  begin  to  act  on  it,  the  iodide  com- 
mences changing,  and  is  capable  of  being  whitened  by  mercury.  Step  by  step  this 
process  goes  on,  an  increased  whiteness  resulting  from  the  prolonged  action  or  increased 
brilliancy  of  the  light,  until  a  certain  point  is  gained,  and  now  the  iodide  of  silver  ap- 
parently undergoes  no  farther  visible  change ;  but  another  point  being  gained,  it  begins 
to  assume,  when  mercurialized,  a  pale-blue  tint,  becoming  deeper  and  deeper,  until  it 
at  last  assumes  the  brilliant  blue  of  a  watch-spring.  This  incipient  blueness  goes  un- 
der the  technical  name  of  solarization. 

613.  The  successful  practice  of  the  art  of  Daguerreotyping,  therefore,  depends  on 
limiting  the  action  of  the  sun-ray  to  the  first  moments  of  change  in  the  iodide  ;  for,  if 
the  exposure  be  continued  too  long,  the  high  lights  become  stationary,  while  the  shad- 
ows increase  unduly  in  whiteness,  and  all  this  happens  long  before  solarization  sets  in. 

614.  Let  us  examine  these  important  phenomena  more  minutely.  Having  carefully 
cleaned  and  iodized  a  silver  plate  three  inches  by  four  in  size,  it  is  to  be  kept  in  the 
dark  an  hour  or  two. 

615.  By  a  suitable  set  of  tin  foil  screens,  rectangular  portions  of  its  surface,  half  an 
mch  by  one  eighth,  are  to  be  exposed  at  a  constant  distance  to  the  rays  of  an  argand 
gas-burner  (the  one  I  have  used  is  a  common  twelve-holed  burner),  the  first  portion 
being  exposed  fifteen  seconds,  the  second  thirty  seconds,  the  third  forty-five  seconds, 
the  fourth  sixty  seconds,  &c.,  &c. 

616.  We  have  thus  a  series  of  discs  or  spaces  upon  the  plate  {a,  b,  c,  d,fig.  96),  each 
of  which  has  been  affected  by  known  quantities  of  light ;  h  being  affected  twice  as 
much  as  a,  having  received  a  double  quantity  of  light ;  c  thrice  as  much  as  a,  having 
received  a  triple  quantity,  &c.,  &c. 

617.  The  plate  now  is  exposed  to  the  vapour  of  mercury  at  170°  Fah.  for  ten  min- 
utes ;  the  spaces  or  discs  all  come  out  in  their  proper  order,  and  nothing  remains  but 
to  remove  the  iodine. 

618.  An  examination  of  one  of  these  plates  thus  prepared  shows  us*  that,  commen 
cing  with  the  first  space  a,  we  discover  a  gradual  increase  of  whitening  effect  until  we 
reach  the  seventh  ;  that  a  perfect  whiteness  is  there  attained  ;  that,  passing  on  to  the 
sixteenth,  no  increase  of  whitening  is  to  be  perceived,  although  the  quantities  of  light 
that  have  been  incident  and  absorbed  have  been  continually  increasing  ;  but  as  soon 
as  the  light  thus  latent  has  reached  a  certain  quantity,  visible  decomposition  sets  in, 
indicated  by  a  blueness,  and  the  sensitive  surface  once  more  renders  evident  the  incre- 
ments of  incident  light. 

619.  Or,  by  presenting  a  plate  covered  with  a  screen  to  a  sky  that  is  clear  or  uni- 
formly obscured,  and  with  a  regular  motion,  withdrawing  the  screen  deliberately  from 
one  end  to  the  other,  and  then  suddenly  screening  the  whole,  it  is  plain  that  those 
parts  first  uncovered  will  have  received  the  greatest  quantity  of  light,  and  the  others 
less  and  less.    On  mercurializing,  it  will  be  seen  that  a  stain  will  be  evolved  on  the 

*  It  is  impossible  to  represent  these  changes  in  a  drawing,  which  is  simply  black  and  white  ;  it  will  be  understood  that 
the  characteristic  distinction  of  the  spaces,  from  the  sixteenth  to  the  twentieth,  for  example,  depends  on  their  assuming  a 
blue  tint,  which  continually  deepens  in  intensity. 


OPTICAL  QUALITIES  CONTROL  CHEMICAL  ACTION.  153 

plate,  as  is  represented  in^/ig.  07  ;  from  a  to  b  the  changes  have  been  successive ;  from 
6  to  c  no  variation  in  the  amount  of  whitening  is  perceptihle  ;  at  d  solarization  is  com- 
mencing, which  becomes  deeper  and  deeper  to  the  end,  e,  of  the  stain. 

620.  The  plate  from  which  the  drawing  of  Jig.  97  is  taken  gives,  from  a  to  b,  ten 
parts,  from  b  to  c  seventeen  parts,  from  d  to  e  twelve  parts  ;  we  perceive,  therefore,  how 
large  an  amount  of  light  is  absorbed,  and  its  effects  rendered  latent,  between  the  maxi- 
mum of  whiteness  being  gained  and  solarization  setting  in. 

621.  4th.  Thai  it  depends  on  the  chemical  nature  of  the  ponderable  material  what 
rays  shall  be  absorbed. 

I  had  prepared  a  number  of  observations  in  proof  of  this,  very  much  of  the  same  kind 
as  those  which  have  some  time  ago  been  published  in  the  Philosophical  Transactions 
by  Sir  J.  Herschel.  These  refer  chiefly  to  the  variable  lengths  of  the  stains,  impress- 
ed by  the  prismatic  solar  spectrum  on  different  chemical  bodies,  and  the  points  of  max- 
imum action  noticed  in  them.  For  the  present,  I  content  myself  with  referring  to  that 
excellent  memoir  for  proofs  substantiating  this  proposition. 

622.  5th.  That  ivhile  the  specific  rays  thus  absorbed  depend  upon  the  chemical  nature 
of  the  body,  the  absolute  amount  is  regulated  by  its  optical  qualities,  sucIl  as  depend 
on  the  condition  of  its  surfaces  and  interior  arrangement. 

623.  I  took  a  pohshed  silver  plate,  and,  having  exposed  it  to  the  vapour  of  iodine, 
found  that  it  passed  through  the  following  changes  of  colour :  1st,  lemon  yellow  ;  2d, 
golden  yellow ;  3d,  reddish  yellow ;  4th,  blue ;  5th,  lavender ;  6th,  metallic  ;  7th,  yel- 
low ;  8th,  reddish ;  9th,  green,  &c.,  &c.,  the  differences  of  colour  being  produced  by 
the  differences  of  thickness  in  the  film  of  iodide,  and  not  by  any  difference  of  chemical 
quahty. 

624.  It  is  a  common  remark,  originally  made  by  M.  Daguerre,  that  of  these  dif- 
ferent tints  that  marked  2  is  the  most  sensitive,  and  photogenic  draughtsmen  generally 
suppose  that  the  others  are  less  eflficient  from  the  circumstance  of  the  film  of  iodide 
being  too  thick.  Some  suppose,  indeed,  that  the  first  yellow  alone  is  sensitive  to  light. 
We  shall  see,  in  a  few  moments,  that  this  is  very  far  from  being  the  case. 

625.  Having  brought  nine  different  plates  to  the  different  colours  just  indicated,  I  re- 
ceived on  each  the  image  of  a  uniform  gas  flame  in  the  camera,  treating  all  as  nearly 
alike  as  the  case  permitted.  I  readily  found  that  in  No.  1  there  was  a  well-marked 
action  ;  No.  2,  still  stronger,  but  that  the  rays  had  less  and  less  influence  down  to  No. 
6,  in  which  they  appeared  to  be  almost  without  action  ;  but  in  No.  7  they  had  recovered 
their  original  power,  being  as  energetic  as  in  No.  2,  and  from  that  declining  again  ;  this 
is  shown     fig.  98. 

626.  Hence  we  see  that  the  sensitiveness  of  the  iodide  of  silver  is  by  no  means  con- 
stant ;  that  it  observes  periodical  changes,  which  depend  on  the  optical  qualities  of  the 
film,  and  not  on  its  chemical  composition ;  and  that  by  bringing  the  iodide  into  those 
circumstances  that  it  reflects  the  blue  rays,  we  greatly  reduce  its  sensitiveness,  and  still 
more  so  when  we  adjust  its  thickness  so  as  to  give  it  a  gray  metallic  aspect.  But  the 
moment  we  go  beyond  this,  and  restore  by  an  increased  thickness  its  original  colour, 

U 


154 


THE  ACTIVE  RAYS  ARE  ABSORBED, 


we  restore  also  its  sensitiveness.  Here,  then,  in  this  remarkable  result,  we  again  perceive 
a  corroboration  of  our  first  proposition. 

627.  I  may,  however,  observe  in  passing,  that  akhough  I  am  describing  these  ac- 
tions as  if  there  was  an  actual  absorption  of  the  rays,  and  that  films  on  metallic  plates 
exhibit  colours,  not  through  any  mechanism  like  interference,  but  simply  because  they 
have  the  power  of  absorbing  this  or  that  ray,  there  is  no  difficulty  in  translating  these 
observations  into  the  language  of  that  hypothesis.  When  the  diffracted  fringes  given 
by  a  hair  or  wire  in  a  cone  of  diverging  light  are  received  on  these  plates,  correspond- 
ing marks  are  obtained,  a  dark  stripe  occupying  the  place  of  a  yellow  fringe,  and  a 
white  that  of  a  blue.  I  found,  more  than  four  years  ago,  that  this  held  in  the  case  of 
bromide  of  silver  paper  (446),  and  have  since  verified  in  a  more  exact  way  with  this 
French  preparation.  Similar  phenomena  of  interference  may  be  exhibited  with  the 
chloride  of  silver. 

628.  We  have  it,  therefore,  in  our  power  to  exalt  or  depress  the  sensitiveness  of  any 
compound  by  changing  its  optical  conditions.  Until  now,  it  has  been  supposed  that 
the  amount  of  change  taking  place  in  different  bodies  by  the  action  of  the  rays  of 
light,  depended  wholly  on  their  chemical  constitution,  and  hence  comparisons  have 
been  instituted  as  to  the  relative  sensitiveness  of  the  chlorides,  bromides,  oxides,  and 
iodides  of  silver,  &c.  But  it  seems  this  liability  to  change  depends  also  on  other  prin- 
ciples, which  being  liable  to  variation,  the  sensitiveness  of  a  given  body  varies  with 
them.  Thus  this  very  iodide  of  silver,  when  in  a  thin  yellow  film,  is  decomposed  by 
the  feeblest  rays  of  a  taper,  and  even  moonlight  acts  with  energy ;  yet  simply  by  alter- 
ing the  thickness  of  its  film,  it  becomes  sluggish,  blackening  in  the  sunlight  tardily,  and 
recovers  its  sensitiveness  again  on  recovering  its  yellow  hue. 

629.  We  have  now  no  difficulty  in  understanding  how,  in  the  preparation  of  ordi- 
nary sensitive  paper,  great  variations  ensue  by  modifying  the  process  slightly,  and  how, 
even  on  a  sheet  which  is  apparently  washed  uniformly  over,  large  blotches  appear 
which  are  either  inordinately  sensitive  or  not  sensitive  at  all.  If,  without  altering  the 
chemical  composition  of  a  film  on  metallic  silver,  or  even  its  mode  of  aggregation,  such 
striking  changes  result  by  difference  of  thickness,  how  much  more  may  we  expect  that 
the  great  changes  in  molecular  condition,  which  apparently  trivial  causes  must  bring 
about  on  sensitive  paper,  should  elevate  or  depress  its  capability  of  being  acted  on  by 
light  !  If  I  mistake  not,  it  is  upon  these  principles  that  an  explanation  is  to  be  given 
of  the  successful  modes  of  preparation  which  Mr.  Talbot  and  Mr.  Hunt  have  descri- 
bed, and  the  action  of  the  mordants  of  Sir  John  Herschel. 

630.  I  therefore  infer, 

6th.  That  the  sensitiveness  of  amj  given  preparation  depends  on  its  chemical  na- 
ture and  its  optical  qualities  conjointly,  and  that  it  is  possible  to  exalt  or  diininish  the 
sensitiveness  of  a  given  co/npound  hy  changing  its  optical  relations. 

631.  7th.  That,  as  when  radiant  heat  falls  on  the  surface  of  an  opaque  hodij,  the 
rays  reflected  are  complementary  in  number  to  those  that  are  absorbed,  so  in  the  case  of 
a  sensitive  preparation,  the  rays  reflected  are  complementary  in  number  to  tJwse  that  are 
absorbed. 


I 


AND  THE  COMPLEMENTARY  REFLECTED.  255 

632.  This  important  proposition  I  prove  in  the  following  way  :  I  take  a  plate,  A  G, 
Jig.  99,  three  inches  by  four,  and  by  partially  screening  its  surface,  while  in  the  act  of 
iodizing,  with  a  proper  piece  of  flat  glass,  I  produce  upon  it  five  transverse  bands,  b, 
c,d,e,  f;  the  fifth, which  has  been  longest  exposed,  is  of  a  pale  lavender  colour  ;  the 
fourth,  a  bright  blue  ;  the  third,  a  red ;  the  second,  a  golden  yellow ;  and  the  first,  un- 
iodized  metal ;  the  object  of  this  arrangement  being  to  expose,  at  the  same  time  and 
on  the  same  plate,  a  series  of  films  of  ditferent  colours  and  of  different  thickness,  and  to 
examine  the  action  of  the  rays  impinging  on  them,  and  the  rays  reflected  by  them. 

633.  Having  prepared  a  second  plate,  B,  and  iodized  it  uniformly  to  a  yellow,  I 
deposited  it  in  the  camera,  and  now  placing  the  first  plate,  A  G,  so  that  the  rays 
coming  on  it  through  the  window  from  the  sky  shall  be  specularly  reflected  to  the 
object-glass  of  the  camera,  and  the  image  of  A  G  form  upon  B,  I  allow  the  exposure  to 
continue  until  the  yellow  of  A  G  is  beginning  to  turn  brown ;  then  I  shut  the  camera 
and  mercurialize  both  plates. 

634.  In  consequence  of  what  has  been  said  (625),  it  will  be  readily  understood,  that 
of  the  bands  on  A  G,  the  first  one,  which  is  the  bare  metal,  does  not  whiten  in  the  mer- 
cury vapour ;  the  second,  which  is  yellow,  mercurializes  powerfully  ;  the  tiiird,  which 
is  red,  is  less  affected  ;  the  fourth,  which  is  blue,  still  less ;  and  the  fifth,  which  is  lav- 
ender, hardly  perceptibly. 

635.  But  the  changes  on  B,  which  have  been  brought  about  by  the  rays  reflected 
from  A  G,  are  precisely  the  converse;  the  band,  which  is  the  image  of  h,  is  mercurial- 
ized powerfully;  that  of  c  is  untouched,  and  absolutely  black,  d  faintly  stained,  e  whiten- 
ed, and /  mercuriaUzed,  but  Uttle  less  than  h. 

636.  It  follows  from  this,  that  a  white  stripe  on  B  corresponds  to  a  black  one  on 
A  G,  and  the  converse ;  and  for  the  depth  of  tint  of  the  intermediate  stripes,  those  of 
the  one  are  perfectly  complementary  to  the  corresponding  ones  of  the  other. 

637.  By  the  aid  of  these  results,  we  are  now  able  to  give  an  account  of  the  varia- 
bility of  sensitiveness  in  photogenic  preparations ;  the  yellow  iodide  of  silver  is  exces- 
sively sensitive,  because  it  absorbs  all  the  chemical  rays  that  can  disturb  it,  while  the 
lavender  is  insensitive,  because  it  reflects  them.  Under  this  point  of  view,  sensitive- 
ness, therefore,  is  directly  as  absorption,  and  inversely  as  reflexion. 

638.  The  superiority  of  Daguerre's  preparation  over  common  sensitive  paper  may 
now  be  readily  understood.  It  absorbs  all  the  rays  that  can  affect  it,  but  the  chloride 
of  silver,  spread  upon  paper,  reflects  many  of  the  active  rays.  The  former,  when  pla- 
ced in  the  camera,  gives  rise  to  no  reflexions  that  can  be  injurious  ;  the  latter  fills  it 
with  active  light,  and  stains  the  proof  all  over.  Hence  the  Daguerreotype  has  a  sharp- 
ness and  mathematical  accuracy  about  its  lines,  and  a  depth  in  its  shadows,  which  is 
unapproacliable  by  the  other.  Moreover,  the  translucency  of  the  white  chloride  of  sil- 
ver, as  well  as  its  high  reflecting  power,  permits  of  particles  lying  out  of  the  lines  of 
light  being  affected,  the  luminous  agent  being  diffused  in  the  paper. 

639.  The  fact,  therefore,  that  a  given  compound  remains  unchanged  even  in  the 
direct  rays  of  the  sun,  is  no  proof  that  light  cannot  decompose  it ;  it  may  reflect  or 
transmit  the  active  rays  as  fast  as  it  receives  them.    It  results  from  this,  that  optical 


156 


ON  SPECTRAL  IMAGES  AND  LATENT  LIGHT. 


forces  can  control,  and  even  check  the  play  of  chemical  affinities.  While  thus  it  aj)- 
pears  that  there  are  points  of  analogy  between  this  chemical  agent  and  radiant  heat, 
we  must  not  too  hastily  infer  that  the  laws  which  regulate  the  one  obtain  exclusively 
also  with  the  other.  As  is  well  known,  there  are  striking  analogies  between  radiant 
heat  and  light,  but  ihere  are  also  points  of  difference ;  the  convertibility  of  heat  of  one 
degree  of  refrangibility  to  another  does  not  occur  with  light ;  there  are  also  dissimili- 
tudes in  the  phenomena  of  radiation  and  its  consequences.  I  do  not  doubt  that  what 
has  been  communicated  in  this  memoir  will,  by  tbe  researches  of  others,  be  greatly 
extended ;  but  it  is  not  to  be  expected  that  a  complete  parallel  can  be  run  between 
radiant  heat  and  the  chemical  rays,  any  more  than  between  radiant  heat  and  light. 

640.  From  the  phenomena  of  the  interference  of  these  rays,  of  the  sensitiveness  or 
non-sensitiveness  of  the  same  cbemical  compound  being  determined  merely  by  the  fact 
of  its  thickness  or  thinness,  these,  and  many  other  similar  results,  ohviously  depending 
upon  mechanical  principles,  it  seems  to  me  that  very  powerful  evidence  may  be  drawn 
against  the  materiality  of  light,  and  its  entering  into  chemical  union  with  ponderable 
atoms.  Those  philosophers  who  have  endeavoured  to  prove  the  undulatory  theory, 
will  probably  find,  in  studying  these  subjects,  cogent  evidence  in  favour  of  their  doctrines. 


Note  added  to  the  preceding  Chapter. 

ON   CERTAIN  SPECTRAL  APPEARANCES,  AND  ON  THE   DISCOVERY   OF  LATENT  LIGHT. 

(^Reing  a  Letter  to  the  Editors  of  the  London  and  Edinburgh  Philosophical  Magazine,  inserted,  in  that  Journal,  November,  1842.) 

641.  Gentlemen — If  there  be  a  thing  in  which  I  have  a  disinclination  to  engage, 
it  is  controversy  of  a  personal  kind  with  scientific  fellow-labourers.  But  as,  you  well 
know,  it  ordinarily  happens  that  there  is  no  other  gain  to  philosophers  beyond  the 
mere  credit  of  their  discoveries,  they  may  be  forgiven  for  reluctantly  endeavouring  to 
secure  this,  their  only  reward. 

642.  I  have  recently  returned  from  a  long  journey,  undertaken  for  the  purpose  of 
making  trials  on  the  sunlight  in  lower  latitudes,  and  am  surprised  to  see  in  the  reports 
that  have  reached  this  country  of  the  Proceedings  of  the  British  Association,  certain 
announcements  received  from  Professor  Bessel,  of  phantoms  which  can  be  produced 
on  surfa(;es  by  mercury  vapour,  by  the  breath,  and  other  means,  as  though  the  thing 
were  neiv.  Years  ago,  if  you  look  in  your  own  Journal  (February,  1840,  p.  84;  Sept., 
1840,  p.  218  ;  Sept,  1841,  p.  198,  199),  you  will  find  tliat  I  had  published  facts  of 
the  kind  ;  spectral  appearances,  that  could  be  revived  on  metals,  glass,  and  other  bodies, 
by  the  breath,  by  vapour  of  camphor,  by  mercury  vapour,  &c.  The  very  purpose  for 
which  I  described  them  was  the  striking  resemblance  of  some  of  them  to  Daguerre- 
otype images.  I  have  repeatedly  shown  that,  by  placing  a  coin  or  any  other  objeci 
on  iodized  silver,  in  the  dark,  the  vapour  of  mercury  will  bring  out  a  representation  of 
it.    And  in  one  of  the  papers  just  quoted,  the  condition  under  which  camera  images 


1 


THE  SOLAR  SPECTRUM  ON  A  DAGUERREOTYPE  PLATE. 


can  be  reproduced  on  a  silver  pl-ate,  even  after  the  plate  has  been  rubbed  with  rotten- 
stone,  is  described, 

643.  I  have  farther  seen  (Literary  Gazette,  July  23, 1842,  Paris  letter),  that  the  fact 
that  light  becomes  latent  in  bodies,  after  the  manner  of  heat,  was  announced  in  France 
as  a  new  and  important  discovery  of  Professor  Moser  of  Konigsburg.  In  your  own 
Journal,  more  than  a  year  ago,  you  printed  a  long  paper  written  by  me  on  this  very 
topic  (September,  1841,  p.  196,  204,  205,  206),  not  merely  announcing  the  fact,  but 
giving  rude  estimates  of  the  amounts:  more  exact  numerical  determinations  I  have 
now  nearly  ready  for  the  press. 

644.  But  I  will  trouble  you  no  farther  with  these  private  matters,  simply  hoping 
that  your  numerous  readers,  who  feel  an  interest  in  such  things,  will  turn  for  them- 
selves to  the  pages  I  have  quoted. 

645.  The  accompanying  photographic  impression  of  the  solar  spectrum,  which  I 
will  thank  you  to  give  to  Sir  John  Herschel,  was  obtained  in  the  south  of  Virginia  : 
probably  you  can  make  nothing  like  it  in  England:  the  sunlight  here,  in  New- York, 
wholly  fails  to  give  any  such  result.  It  proves  that,  under  a  brilliant  sun,  there  is  a 
class  of  rays  commencing  precisely  at  the  termination  of  the  blue,  and  extending  be- 
yond the  extreme  red,  which  totally  and  perfectly  arrest  the  action  of  the  light  of  the 
sky.  This  impression  was  obtained  when  the  thermometer  was  96°  Fahr.  in  the  shade, 
and  the  negative  rays  seem  almost  as  effective  in  protecting  as  the  blue  rays  are  in  de- 
composing iodide  of  silver. 

646.  The  most  remarkable  part  of  the  phenomenon  is,  that  the  same  class  of  rays 
makes  its  appearance  again  beyond  the  extreme  lavender  ray.  Sir  John  Herschel  has 
already  stated,  in  the  case  of  bromide  of  silver,  that  these  negative  rays  exist  low  down 
in  the  spectrum.  This  specimen,  however,  proves  that  they  exist  at  both  ends,  and 
do  not  at  all  depend  on  the  refrangibility.  It  was  obtained  with  yellow  iodide  of  sil- 
ver, Daguerre's  preparation,  the  time  of  exposure  to  the  sun  fifteen  minutes. 

647.  In  this  impression,  six  different  kinds  of  action  may  be  distinctly  traced  by  the 
different  effects  produced  on  the  mercurial  amalgam.  These,  conniiencing  with  the  most 
refrangible  rays,  may  be  enumerated  as  follows  :  1st,  protecting  rays  ;  2d,  rays  that  whi- 
ten ;  3d,  rays  that  blacken ;  4th,  rays  that  whiten  intensely  ;  5th,  rays  that  whiten  very 
feebly;  6th,  protecting  rays. 

648.  It  is  obvious  we  could  obtain  negative  photographs  by  the  Daguerreotype  pro- 
cess, by  absorbing  all  the  rays  coming  from  natural  objects,  except  the  red,  orange,  yel- 
low, and  green,  allowing,  at  the  same  time,  diffused  daylight  to  act  on  the  plate. 

649.  This  constitutes  a  great  improvement  in  the  art  of  photography,  because  it  per- 
mits its  application,  in  a  negative  way,  to  landscapes.  In  the  original  French  plan, 
the  most  luminous  rays  are  those  that  have  least  effect,  while  the  sombre  blue  and  violet 
rays  produce  all  the  action.  Pictures  produced  in  that  way  never  can  imitate  the  or- 
der of  light  and  shadow  in  a  coloured  landscape. 

650.  If  it  should  prove  that  the  sunlight  in  tropical  regions  differs  intrinsically  from 
ours,  it  would  be  a  very  interesting  physical  fact.  There  are  strong  reasons  to  believe 
it  is  so.    The  Chevalier  Frederichstal,  who  travelled  in  Central  America  for  the  Prus- 


168 


ANALOGIES  BETWEEN  THE  CHEMICAL  RAYS  AND  HEAT. 


sian  government,  found  very  long  exposures  in  the  camera  needful  to  procure  impres- 
sions of  the  ruined  monuments  of  the  deserted  cities  existing  there.  This  was  not  due 
to  any  defect  in  his  lens ;  it  was  a  French  achromatic,  and  I  tried  it  in  this  city  with 
him  before  his  departure.  The  proofs  which  he  obtained,  and  which  he  did  me  the 
favour  to  show  me  on  his  return,  had  a  very  remarkable  aspect.  More  recently,  in  the 
same  country,  other  competent  travellers  have  experienced  like  difficulties,  and,  as  I 
am  informed,  failed  to  get  any  impressions  whatever.  Are  these  difficulties  due  to  the 
antagonizing  action  of  the  negative  rays  upon  the  positive. 


CHAPTER  XIIL 

ON  A  NEW  IMPONDERABLE  SUBSTANCE,  AND   ON  A  CLASS  OF  CHEMICAL   RAYS  ANALOGOUS 

TO   THE    RAYS   OF   DARK  HEAT. 

{From  the  London,  Edinburgh,  and  Dublin  Philosophical  Magazine  for  December,  1842.) 

Contents  :  Analogies  between  the  Chemical  Rays  and  Heat. — Neio  Nomenclature  pro- 
posed.—  Titlwnic  Rays. — Independence  of  Tithonic  Rays  and  Light. — Independence 
of  Tithonic  Rays  and  Heat. — Dark  Tithonic  Rays. 

651.  In  chapter  twelve  I  have  pointed  out  several  analogies  which  may  be  observed 
between  the  phenomena  of  the  chemical  rays  and  those  of  radiant  heat. 

652.  In  this  memoir  it  is  my  intention  to  show  still  more  striking  points  of  analogy, 
and  also  to  direct  the  attention  of  chemists  to  equally  striking  points  of  discordance. 

653.  It  will  be  seen,  from  the  remarkable  facts  detailed  in  this  paper,  that  we  are 
now  forced  to  recognise  the  existence  of  a  new  imponderable  agent,  analogous  in  many 
of  its  properties  to  light,  heat,  and  electricity,  yet  differing  as  much  from  them  all  as 
they  do  from  one  another. 

654.  So  far  as  chemical  analogies  can  direct  us,  there  does  not  appear  anything  un- 
philosophical  in  the  supposition  of  the  existence  of  many  imponderable  agents  analo- 
gous to  those  already  known.  The  progress  of  science  has  indeed  tended  in  different 
directions  in  the  cases  of  the  imponderable  and  ponderable  bodies.  Among  the  former, 
we  have  successively  seen  the  agents  that  are  concerned  in  galvanic  phenomena  and 
those  of  magnetism  merged  into  electricity  ;  but  the  ponderable  bodies,  especially  those 
of  a  metallic  kind,  have  greatly  increased  in  number,  though,  so  far  as  their  more  obvi- 
ous physical  properties  are  concerned,  the  differences  of  many  ar.e  almost  undistinguish- 
able.  We  have  thus  found  it  necessary  to  invert  the  maxims  of  the  early  cultivators 
of  chemistry,  who  extended  the  number  of  ethereal  agents  very  greatly,  and  believed 
that  all  metals  and  other  ponderable  principles  were  modifications  of  one  or  two  pri- 
mordial and  elementary  forms. 

655.  Centuries  ago  it  was  discovered  that  the  sun's  light  had  the  property  of  effect- 
ing chemical  changes  in  bodies,  and  it  is  stated  that  Scheele  first  noticed  that  this 


NEW  i\0:vIL\\CLATliUE  I'iiOPOtjED.  259 

property  was  mainly  due  to  the  violet  rays.  Seebeck  observed  that  chloride  of  silver, 
exposed  to  the  spectrum,  varied  its  colour  with  the  colour  of  the  space  iu  which  it  was 
held,  and  during  the  present  century  a  very  large  amount  of  new  observations  has  been 
accumulated.    A  new  art.  Photography,  has  come  into  existence. 

G56.  The  general  supposition  that  obtains  is,  that  the  effects  in  question  are  due  to 
the  rays  of  light  ;  hence  all  the  words  that  have  been  introduced  into  use  have  refer- 
ence to  that  supposition;  such  words  as  photography,  photology,  photometer,  are  derived 
from  this  erroneous  hypothesis,  and  lead  us  to  confound  together  things  which  ought 
to  be  kept  essentially  distinct. 

657.  As  it  is  the  object  of  this  paper,  and  others  which  I  shall  shortly  publish,  to  call 
the  attention  of  chemists  to  the  agent  that  is  involved  in  photographic  results  as  a 
clearly-established  and  new  imponderable  substance,  possessing  striking  analogies  with 
light  and  heat,  yet  differing  as  much  from  them  both  as  they  do  from  each  other,  I  am 
induced  to  propose  for  it  a  proper  name,  and  to  endeavour  to  establish  for  it  a  nomen- 
clature that  shall  be  free  from  ambiguity,  and  keep  the  description  of  its  phenomena 
separate  from  those  of  light.  While,  therefore,  I  show  that  it  undergoes  radiation, 
reflexion,  refraction,  polarization,  absorption,  interference,  &c.,  under  the  laws  to  which 
its  radiant  companions,  light  and  heat,  are  subject,  I  wish  to  claim  for  it  a  separate  and 
independent  existence,  to  introduce  it  into  the  natural  family  of  imponderable  agents, 
with  light,  heat,  and  electricity.  In  that  family  it  stands  as  the  fourth  member.  Is 
there  any  reason  that  the  progress  of  knowledge  should  not  make  known  to  us  multi- 
plied forms  of  imponderable  substances  as  well  as  of  ponderable  matters  ?  This  agent 
differs  from  light  and  heat  as  much  as  lead  differs  from  zinc  or  tin. 

658.  When  novel  effects,  brought  about  by  novel  causes,  are  met  with,  the  purposes 
of  science  require  new  corresponding  terms.  In  the  case  of  the  chemical  rays  of  light 
it  is  so.  I  have  experienced  the  need  of  a  nomenclature  of  the  kind  from  my  earliest 
experiments.  It  is  a  rule  of  which  modern  philosophers  know  the  value,  that  such 
names  ought  to  be  free  from  all  attending  hypothesis  ;  for  if  this  be  not  complied  with, 
it  soon  comes  to  pass,  as  knowledge  advances,  that  terms  involving  theoretical  ideas 
lose  much  of  their  significance. 

659.  The  chemical  rays  are  associated  with  the  rays  of  light,  accompanying  them 
in  all  their  movements,  originating  with  them,  and,  unless  disturbed,  continuing  to  exist 
along  with  them.  But  should  a  compound  beam  like  this  fall  upon  a  sensitive  surface, 
the  chemical  rays  sink  into  it,  as  it  were,  and  lose  all  their  force,  and  the  rays  of  light 
are  left  alone.  Photographic  results  thus  resulting  from  the  reposing  of  the  chemical 
rays  on  the  sensitive  surface  are  not,  however,  in  themselves  durable,  as  will  be  shown 
in  this  paper,  for  the  rays  escape  away  under  some  new  form. 

660.  "  Tithonus  was  a  beautiful  youth  whom  Aurora  fell  in  love  with  and  married 
in  heaven.  The  fates  had  made  him  immortal;  but,  unlike  his  bride,  in  the  course  of 
events  he  became  feeble  and  decrepit,  and,  losing  all  his  strength,  was  rocked  to  sleep 
in  a  cradle.  The  goddess,  pitying  his  condition,  metamorphosed  him  into  a  grasshop- 
p  e  r. " — Mythological  Dictionary. 

661.  The  fact  and  the  fable  agree  pretty  well;  and,  indeed,  the  playful  coincidence 


TITHONIC  RAYS.— INDEPENDENCE  OF  TITHONIC  RAYS  AND  LIGHT. 


might  be  carried  much  farther.  The  powers  of  photography,  which  bring  architectural 
remains  and  the  forms  of  statuary  so  beautifully  and  impressively  before  us,  might  seem 
to  be  prefigured  by  the  speaking  image  of  the  son  of  Tithorms  and  Aurora,  that  was 
to  be  seen  in  the  deserts  of  Egypt.  And  besides  this,  such  words  as  tithouoscope, 
tithonometer,  tithonography,  tithonic  effect,  diatithonescence,  are  musical  in  an  Eng- 
lish ear.  In  this  paper  I  shall,  therefore,  use  the  term  tithonicity  and  its  derivatives  in 
the  same  manner  that  we  use  electricity  and  its  derivatives. 

662.  This  comumnication  takes  up  the  consideration  of  three  distinct  facts  : 
1st.  The  proof  of  the  physical  independence  of  tithonicity  and  light. 

2d.  The  proof  of  the  physical  independence  of  tithonicity  and  heat. 

3d.  The  proof  of  the  existence  of  dark  tithonic  rays,  analogous  to  the  rays  of 
DARK  heat.  Under  this  head  it  will  be  shown  that  Tithonicity,  like  heat,  enters  tran- 
siently into  bodies,  producing  specific  changes  on  them,  and  then  slowly  and  invisibly 
RADIATES  away.  And  the  physical  constitution  of  the  new  class  of  rays  thus  formed 
is  entirely  different  from  that  of  rays  that  come  from  incandescent  sources  :  a  distinction 
having  a  striking  analogy  in  the  case  of  heat.  Tithonicity  becomes  transiently  and 
Ijermanenthj  latent  in  bodies. 

663.  Figure  100  serves  to  show  that  by  the  agency  of  absorbent  media  we  may 
detect  the  existence  of  tithonic  rays  in  every  part  of  the  spectrum  unaccompanied  by 
light.  The  results  there  projected  were  obtained  by  an  arrangement  such  as  that  in 
jig.  101.  From  a  heliostat  mirror,  a  a,  a  beam  of  the  sun's  light  was  thrown  in  a  hor- 
izontal position,  and  falling  on  a  screen,  h  h,  a  portion  of  it  passed  through  a  circular 
aperture  one  fourth  of  an  inch  in  diameter.  At  the  distance  of  ten  or  twelve  feet  it  fell 
on  a  glass  trough  c  c,  with  parallel  faces,  into  which  any  coloured  solution  could  be 
placed;  immediately  behind  the  trough  there  was  a  double  convex  lens,  dd,  of  three 
feet  focal  length,  and  between  them  a  second  screen,//,  with  an  aperture  correspond- 
ing to  the  centre  of  the  lens,  half  an  inch  in  diameter.  Behind  the  lens  was  situated 
a  prism  of  flint  glass,  e,  which  effected  the  dispersion  of  the  incident  beam.  Now  the 
lens  not  being  achromatic,  the  screen  r  v  had  to  be  placed  in  an  inclined  position  in 
order  to  obtain  a  neat  spectrum-image  of  the  hole  in  h  b,  and  this  was  attended  with 
the  great  advantage  of  elongating  the  total  length  of  the  spectrum,  and,  therefore,  in- 
creasing the  measures.  In  order  to  obtain  sensitive  surfaces  of  great  delicacy,  the  silver 
plates  were  first  iodized  lightly,  and  then  exposed  to  the  vapour  of  bromine  until  they 
attained  a  full  golden  yellow. 

664.  In  fig.  100,  the  line  No.  1  represents  the  risible  colorific  spectrum  ;  it,  with  No.  2, 
serves  as  an  index  of  comparison  for  all  the  others.  No.  2  represents  the  effect  of  a 
spectrum  that  has  not  undergone  the  action  of  any  absorbent  medium  on  the  bromo- 
iodized  plate;  the  extreme  red  tinges  the  plate  white,  the  extreme  violet,  brown,  and  all 
the  intermediate  space  is  of  a  rich  brownish  violet,  with  a  point  of  maximum  action 
nearly  in  its  centre.  The  numerical  subdivisions  commence  with  0  at  the  extreme  red, 
and  are  graduated  on  a  principle  which  I  shall  explain  in  a  future  paper,  which  makes 
the  spectrum  of  different  tithonographists  comparable. 

665.  No.  3  shows  the  spectrum  after  absorption  by  the  persulphocyanide  of  iron,  and 


INDEPENDENCE  OF  TITHONIC  RAYS  AND  HEAT.  261 

its  corresponding  tithonogi-aph.  This  spectrum  is  divided  into  three  portions,  one  of 
which  is  red  and  yellow,  a  second  indigo,  and  a  third  violet.  But  the  tithonograph  ex- 
hibits an  action  far  beyond  the  extreme  red,  half  way  through  the  dark  space  that  in- 
tervenes in  the  middle  of  the  spectrum,  both  ends  of  this  lower  part  projecting  into 
dark  spaces  ;  while  the  indigo  ray,  ordinarily  so  active,  does  not  tithonize  at  all. 

666.  Without  going  into  a  long  descriptive  detail  of  the  comparison  of  different 
spectra  and  their  corresponding  tithonographs,  I  shall  here  sum  up  the  results  which 
may  be  gathered  from  an  inspection  of  the  plate. 

667.  By  the  absorbent  action  of  the  persulphocyanide  of  iron,  we  can  prove  the  ex- 
istence of  invisible  tithonic  rays  beyond  the  extreme  red — invisible  rays  corresponding 
to  the  green.  We  can  also  prove  that  the  indigo-coloured  rays  of  light  may  exist 
without  tithonic  effect. 

668.  By  the  absorbent  action  of  neutral  chloride  of  gold,  we  can  insulate  blue-col- 
oured rays  of  light  that  are  not  tithonic. 

669.  The  green  solution  formed  by  a  mixture  of  bichromate  of  potash,  mnriaric  arid, 
and  alcohol,  enables  us  to  insulate  tithonic  rays  of  the  same  refrangibility  as  the  violet, 
but  unaccompanied  by  any  light. 

670.  The  solution  of  sulphate  of  copper  and  ammonia  enables  us  to  insulate  a  visible 
red  and  yellow  ray  that  are  without  tithonic  power,  and  an  invisible  tithonic  ray  beyond 
the  violet. 

671.  The  solution  of  litmus  enables  us  to  obtain  red  and  green  light  without  action, 
and  an  invisible  tithonic  ray  corresponding  to  the  violet. 

672.  The  solution  of  bichromate  of  potash  enables  us  to  obtain  red  and  orange  light 
without  any  tithonic  effect. 

673.  Such  results  might  be  multiplied  without  end,  for,  indeed,  there  is  scarcely 
an  instance  in  which  spectra  of  rays  that  have  passed  absorbent  media  are  exactly 
coincident  with  their  corresponding  tithonographs.  To  set  the  matter  plainly  before 
the  reader,  the  following  tabular  view,  gathered  from  the  plate,  may  suffice : 


Name  of  Solution. 

Colour  of  LIGHT 

without 
tithonir  effect. 

Invisible  TITHONIC  rays 
correspondintr  in  refran- 

C-lbllity  tn  the 

Persulphocyanide  of  iron  .  . 

Chloride  of  gold  

Chrome  solution  

Sulph.  copper  and  ammonia  . 

Blue. 

Red,  yellow  .... 
Red,  green  .... 
Red,  orange 

Extreme  red,  green 
Violet. 

Extreme  violet. 
Violet. 

Bichromate  of  potash  .... 

From  this,  therefore,  I  infer  the  entire  independence  throughout  the  spcctnun  of  the 
luminous  rays  that  give  to  the  organs  of  vision  the  impression  of  colour  and  the  tithonic 
rays. 

674.  When  I  come  to  describe  the  dark  tithonic  rays  that  are  analogous  to  the  rays 
of  dark  heat,  and  which  are  unaccompanied  by  any  kind  of  light  whatsoever,  no  farther 
doubt  can  be  entertained  on  this  subject.  I  have  also  some  other  proofs  of  a  very 
remarkable  kind,  to  be  described  hereafter,  drawn  from  the  phenomena  exhibited  by 
tithonic  rays  that  have  undergone  polarization. 

675.  Next,  as  to  the  independence  of  these  rays  and  the  rays  of  heat. 

X 


162 


DARK  TITHONIC  RAYS. 


676.  One  of  the  most  striking  proofs  of  this  is  the  facility  with  which  impressions  of 
the  moon's  disc  may  be  obtained  on  Daguerreotype  and  other  sensitive  plates.  Even 
with  lenses  of  comparatively  small  diameter,  and  in  the  space  of  a  few  minutes,  strong 
impressions  of  the  moon's  surface  may  be  taken.  There  is  no  more  difficulty  in  ob- 
taining these  sketches  than  there  is  in  copying  a  building  or  a  statue,  or  any  other  ob- 
ject on  which  the  sun  is  shining.  But  the  moonbeams  have  hitherto  given  no  trace  of 
the  presence  of  heat. 

677.  I  found,  moreover,  by  direct  trial,  that  plates  which  had  been  carefully  prepared, 
so  as  to  be  exceedingly  sensitive,  were  unaffected  by  the  radiant  heat  of  copper  at  any 
temperature  up  to  a  red  heat.  These  dark  rays,  therefore,  have  no  kind  of  effect  on 
such  surfaces.  A  sensitive  plate  may  be  made  so  hot  that  it  cannot  be  touched,  yet  its 
surface  remains  unchanged;  and  even  the  radiant  heat  emitted  by  brightly  incandescent 
bodies  has  no  effect,  as  I  also  proved. 

678.  Lastly,  Proof  of  the  existence  of  darh  tithonic  rays  analogous  to  the  rays  of 

DARK  HEAT. 

679.  The  experiments  now  to  be  described  were  made  with  Daguerreotype  plates 
iodized  at  first  to  a  pale  lemon  yellow,  then  brought  to  a  golden  hue  by  immersion  in 
the  vapour  of  bromine,  and  lastly  exposed  for  a  short  time  to  the  vapour  of  iodine  again. 

680.  Having  exposed  such  a  plate  {fig.  102),  a  h,  to  the  action  of  weak  daylight,  or 
lamplight,  for  a  period  of  time  which  would  cause  it  to  whiten  powerfully  all  over  if  placed 
in  the  vapour  of  mercury,  carry  it  into  a  room  which  is  totally  dark,  and  suspend  at  a  dis- 
tance of  one  eighth  of  an  inch  from  its  surface  a  metallic  screen,  c  d,  the  under  surface 
of  which  is  blackened.  Let  all  remain  in  the  dark  four  or  five  hours,  and  then  remove 
the  sensitive  plate  a  b,  and  expose  it  to  the  vapour  of  mercury.  All  that  portion  of  it 
which  was  not  covered  by  the  screen  c  d  will  undergo  no  change,  but  that  which  was 
beneath  c  d  will  whiten  powerfully. 

681.  From  this  remarkable  result  I  infer  that  the  tithonicity  that  had  originally  dis- 
turbed the  surface  of  the  plate  equally  all  over,  has  escaped  away  from  those  portions 
that  were  uncovered,  but  that  its  escape  has  been  entirely  prevented  by  the  action  of  the 
screen  ;  and  this  must  be  through  radiation,  for  the  screen  is  at  a  distance,  and  has 
never  touched  the  plate.  And,  farther,  that  the  rays  that  do  thus  escape  away  are  abso- 
lutely invisible  to  the  eye. 

682.  Now  suppose  a  piece  of  black  cloth,  placed  in  the  rays  of  the  sun  until  it  has 
become  warm,  were  carried  into  a  cold  room,  and  half  its  surface  screened  by  some 
material,  as  a  piece  of  glass,  at  a  short  distance :  there  cannot  be  a  doubt  that  the  un- 
covered portion  would  cool  fast  by  radiation,  but  the  screened  portion  more  slowly,  for 
its  radiation  would  be  arrested  by  the  glass  plate. 

683.  The  two  cases  are  absolutely  alike. 

684.  Tithonicity,  therefore,  radiates  exactly  after  the  manner  of  heat. 

685.  This  also  furnishes  proof,  in  addition  to  those  I  have  heretofore  given,  that  not 
only  does  tithonicity  become  latent  in  bodies,  but  that  it  becomes  latent  in  two  ways, 
U-ansiently  and  permanently,  exactly  after  the  manner  of  heat. 

686.  The  same  result  is  obtained  when  other  sensitive  surfaces  are  employed,  the 


DARK  TITHONIC  RAYS.  263 

period  of  time  differing  for  different  bodies.  Guided,  therefore,  by  the  analogy  of  heat, 
I  perceive  that  bodies  iiave  a  relation  to  this  imponderable  agent  corresponding  to  that 
of  specific  heat.    It  follows,  therefore,  with  certainty,  that, 

687.  The  specific  titlionicity  of  bodies  is  the  prime  function  on  which  their  sensi- 
tiveness depends.  Under  this  point  of  view  the  sensitiveness  is  inversely  as  the  spe- 
cific tithonicity. 

688.  The  circumstances  under  which  this  experiment  is  made  serve  also  to  show 
that  metallic  bodies  are  non-conductors  of  tithonicity. 

689.  This  contrasts  remarkably  with  their  action  towards  heat. 

690.  Having  exposed  a  sensitive  plate,  a  b,  to  light  until  it  would  whiten  if  mercuri- 
alized, as  before,  and  having  prepared  a  second  (c  d,Jig.  102)  in  total  darkness,  with- 
out allowing  any  light  to  have  access  to  it,  suspend  this  latter  over  the  foruier  at  the 
distance  of  one  eighth  of  an  inch,  so  as  to  cover  it  about  half  Keep  the  two  plates  in 
darkness  for  several  hours,  and  then  mercurialize  both.  That  portion,  a  c,  of  the  first 
not  covered  by  the  second,  will  not  whiten  ;  that  portion  of  the  second,  b  d,  not  cov- 
ered by  the  first,  will  also  remain  unchanged  ;  but  both  on  those  parts  that  have  looked 
towards  each  other  will  whiten. 

691.  From  this  I  infer,  that  the  portion  of  the  first  not  overshadowed  by  the  second 
does  not  whiten,  because  its  tithonicity  escapes  away  under  the  form  of  dark  tithonic 
rays. 

692.  I  also  infer,  that  as  both  plates  are  nearly  equally  whitened  on  those  portions 
of  their  surfaces  that  have  looked  towards  each  other,  the  dark  tithonic  rays  that  have 
escaped  from  the  first  plate,  notwithstanding  their  invisibility,  have  retained  their  pecu- 
liar chemical  force,  and  have  affected  the  second  plate. 

693.  The  analogy  with  heat  is  here  perfectly  observed.  A  hot,  non-conducting  plate, 
set  partially  opposite  a  cold  one,  would  warm  that  plate  on  the  portion  looking  towards 
it,  and,  through  the  consequent  retardation  of  radiation,  would  retain  its  own  heat  to  a 
certain  extent.  But  all  those  portions  unopposed  by  the  cold  plate  would  cool  down 
by  radiation  rapidly. 

694.  This  experiment  proves,  in  a  clear  and  undoubted  manner,  the  total  physical 
independence  of  tithonicity  and  light. 

695.  Hence  the  absolute  necessity  of  some  such  nomenclature  as  that  proposed  ;  the 
chemical  rays  of  fight  is  a  misnomer. 

696.  On  the  surface  of  a  sensitive  plate  that  has  been  suitably  exposed,  as  hereto- 
fore, place  a  fragment  of  perfectly  clean  and  colourless  glass.  Allow  it  to  remain  there 
for  four  or  five  hours  in  a  dark  room,  then  mercurialize,  and  it  will  be  found  that  the 
portion  on  which  the  glass  has  been  placed  will  whiten  powerfully,  but  all  the  rest  will 
remain  unchanged. 

697.  This,  therefore,  proves  that  colourless  glass  is  nearly  opaque  to  the  dark  tithonic 
rays,  a  result  observed,  also,  in  the  case  of  the  dark  rays  of  heat. 

698.  I  made  a  comparative  trial  of  the  relative  permeability  of  colourless  plate-glass 
and  common  writing-paper.  A  sensitive  surface  was  exposed  until  it  had  slightly,  but 
very  plainly  commenced  to  turn  brown.    On  one  portion  I  now  laid  a  piece  of  clear 


f 


164  DARK  TITHONIC  RAYS. 

glass,  and  by  the  side  of  it  a  piece  of  writing-paper;  the  arrangement  was  next  pla- 
ced ill  tlie  dark  for  four  hours;  it  was  then  mercuriahzed  at  160°  Fah.  for  an  hour, 
and  the  resuh  was  very  striking.  Notwithstanding  the  long  exposure  to  the  mercury 
vapour,  all  those  portions  that  had  not  been  covered  were  perfectly  unaffected,  the  por- 
tion that  had  been  covered  by  the  glass  was  of  an  intensely  deep  brown  colour,  but  the 
portion  covered  by  the  paper  was  marked  by  a  distinct,  but  very  faint  white  stain.  It 
was  therefore  plain,  that  from  the  uncovered  portions  all  the  tithonicity  had  radiated 
away  ;  from  the  portions  covered  by  the  writing-paper  the  same  effect  almost  to  the 
same  extent  had  occurred,  the  paper,  however,  slightly  obstructing  the  passage  of  the 
rays  ;  but  radiation  had  been  wholly  prevented  from  those  parts  covered  by  the  colour- 
less glass. 

699.  Writing-paper  is,  therefore,  far  more  permeable  to  the  dark  tithonic  rays  than 
the  purest  plate  glass. 

700.  This  property  it  will  be  hereafter  convenient  to  speak  of  under  the  designation 
of  diatithonescence  or  transtithonescence. 

701.  Blue,  red,  and  yellow  glass  obstruct,  to  a  great  extent,  the  process  of  radiation. 
In  several  trials  it  seemed  as  though  the  yellow  was  more  transparent  than  the  others, 
but  there  was  not  much  difference. 

702.  Transparent  rock-salt  appears  to  hold  very  nearly  the  same  relation  of  diatitho- 
nicity  as  plate  glass. 

703.  In  like  manner,  the  following  substances  in  thin  plates  obstruct  the  radiation  of 
tithonicity  :  Sulphate  of  lime,  beryl,  agate,  rock-crystal,  calc-spar,  mica,  wafers,  metal- 
lic bodies,  cloth  of  cotton,  wood,  ivory,  coloured  glass,  &c.,  &c. 

704.  The  remarkable  results  described  in  the  Philosophical  Transactions  by  Sir 
John  Herschel  (1840,  p.  44),  but  left  by  him  without  any  explanation,  are  of  the 
kind  now  under  discussion.  He  found  that  paper  washed  with  nitrate  of  silver,  if  ex- 
posed to  the  sun  under  a  piece  of  glass,  darkened  much  more  rapidly  than  if  the  glass 
were  away.  This  effect  was  by  no  means  limited  to  that  variety  of  paper,  but  was  ob- 
servable, also,  with  many  other  tithonographic  compounds.  Transparent  minerals,  sucli 
as  topaz,  selenite,  Iceland  spar,  quartz,  produced  the  same  results  as  glass.  But  on 
gloomy  days  the  phenomena  did  not  appear,  a  bright  sunshine  being  apparently  requi- 
site for  their  production.  "  When  a  piece  of  nitrated  paper,  for  instance,  was  rolled 
round  a  cylindrical  surface  of  moderate  convexity,  covered  with  black  velvet,  and  the 
piece  of  glass  laid  gently  in  contact  with  it,  the  effect  of  sunshine  was  exalted  at  the 
line  of  contact;  but  on  either  side  of  that  line,  as  the  interval  increased,  the  influence  of 
the  glass  diminished,  and  at  less  than  half  an  inch  distance  no  difference  could  be  per- 
ceived between  the  impressions  under  the  glass  and  in  the  free  air." 

705.  Now  all  this  is  precisely  what  should  happen  if  the  tithonographic  compound 
radiates  while  it  is  undergoing  decomposition.  The  rays  which  come  from  the  sun  pass 
through  the  glass  with  but  little  loss  from  absorption;  falling  upon  the  nitrate,  they  de- 
compose it,  and  now  it  commences  radiating,  but  the  physical  character  of  these  rays 
is  very  different  from  the  character  they  possessed  before  impinging  on  the  nitrate. 
Now  they  cannot  get  through  the  glass,  before  they  passed  without  difficulty.    So  it  is. 


PRODUCTION  OF  SPECTRAL  APPEARANCES.  165 

precisely,  in  the  case  of  heat.  Much  of  the  heat  of  the  sun  passes  through  plate  glass, 
and  if  it  falls  on  a  dark  surface  that  can  absorb  it,  that  surface  becomes  presently  warm 
and  commences  radiating;  but  the  physical  constitution  of  these  rays  is  changed  :  they 
cannot  get  through  the  glass;  and  if  a  non-conducting  black  surface,  half  covered  by  a 
piece  of  glass  and  half  in  the  free  air,  were  exposed  to  the  sun,  the  covered  half  would, 
for  ihese  obvious  reasons,  become  the  hotter.  For  the  same  reason,  precisely,  in  the 
tithonic  experiment,  the  glass  increases  the  final  effect  by  obstructing  radiation. 

706.  It  is  very  obvious  why  such  effects  cannot  be  produced  on  gloom}'  days.  If 
at  such  times  we  were  to  expose  a  piece  of  black  cloth,  partially  covered  by  glass,  no 
difference  of  temperature  would  be  perceptible  in  its  covered  and  uncovered  portions. 
The  reasons  are  analogous  in  each  case. 

707.  An  experiment,  the  same  in  principle  as  Sir  John  Hrrschel's,  may  be  easily 
made.  Upon  a  sensitive  plate  that  has  been  exposed  a  short  time  to  a  feeble  light, 
place  a  convex  lens  ;  the  arrangement  being  left  for  a  time  in  a  dark  room.  When 
you  have  mercurialized,  you  will  find  a  central  dark  point  corresponding  with  the  point 
of  contact,  and  round  it  a  white  areola  that  shades  gradually  and  imperceptibly  away. 
With  a  lens  with  which  I  have  occasionally  made  this  experiment,  tlie  areola  is  nearly 
an  inch  in  diameter,  the  lens  being  a  double  convex  of  about  two  inches  focus. 


Note  added  to  the  preceding  Chapter. 

ON  THE   RAPID   DETITHONIZING    POWER    OF   CERTAIN    GASES   AND   VAPOURS,  AND   ON  AN 
INSTANTANEOUS   MEANS   OF   PRODUCING   SPECTRAL  APPEARANCES. 

{From  the  London,  Edinburgh,  and  Dullin  Philosophical  Magazine  for  March,  1843.) 

708.  For  some  time  after  I  was  acquainted  with  the  phenomena  mentioned  in  the 
last  chapter,  and  there  referred  to  radiation,  I  was  led  to  attribute  them  to  a  peculiar 
property  which  certain  gases  and  vapours  possess,  of  which  I  propose  now  to  give  a 
detailed  description. 

709.  This  property  is  a  power  of  effecting  a  very  rapid  detithonization  of  surfaces 
♦hat  have  been  poxverfully  tithonized. 

710.  It  affords  the  means  of  instantly  procuring  spectral  appearances  of  external 
forms. 

711.  Referring  now,  in  the  first  place,  to  the  analogies  of  caloric  :  a  body  which  has 
been  warmed  cools  down  to  a  temperature  that  is  in  equilibrium  with  that  of  objects 
around  in  several  different  ways,  by  radiation,  by  currents  in  the  air,  and  often  by  direct 
conduction,  each  of  these  tending  to  produce  the  same  result. 

712.  A  sensitive  surface,  which  has  been  disturbed  by  exposure  to  the  daylight  or 
lamplight,  has  the  quality  of  restoring  itself  to  its  primitive  condition  when  kept  in  the 
dark.  Dagufrre  noticed  this  in  the  case  of  certain  resinous  bodies ;  other  experi- 
menters have  likewise  proved  that  it  takes  place  with  some  varieties  of  the  ordinary 
photogenic  preparations.  I  have  found  that  it  holds  in  the  coloured  films  on  the  sur- 
face of  silver. 


166 


PRODUCTION  OF  SPECTRAL  APPEARANCES. 


713.  Much  of  this  effect  is  due,  as  I  have  endeavoured  in  the  paper  above  quoted  to 
show,  to  a  direct  escape  of  dark  rajs  by  a  process  analogous  to  radiation  ;  but  much 
also  is  due  to  a  hitherto  unknown  power,  possessed  by  electro-negative  gases  and  va- 
pours, which  tends  to  bring  about  the  same  results.  So  powerfully,  indeed,  does  this 
cause  operate,  that,  as  I  have  said,  for  a  length  of  time  I  attributed  all  the  phenomena 
to  it. 

714.  I  proceed  now  to  describe  some  simple  experiments  which  will  bring  this 
matter  clearly  before  the  reader. 

715.  Take  a  bromo-iodized  silver  plate,  expose  it  to  the  light  of  the  sky  or  lamplight 
for  a  length  of  time  sufficient  to  brown  it  sensibly  and  uniformly  all  over.  In  this  state, 
if  it  were  placed  in  the  vapour  of  mercury,  it  would  solarize  or  blacken  in  every  part. 
But,  before  mercurializing,  treat  it  as  follows  :  Lay  upon  it  a  fragment  of  glass,  a  piece 
of  metal,  or  any  other  object ;  immerse  it  for  a  second  or  two  in  a  box  containing  the 
vapour  of  iodine  ;  withdraw  it,  remove  the  little  object,  and  mercurialize  forthwith  ; 
and  now  you  will  find  a  perfectly-formed,  black  spectral  impression  of  the  object, 
whatever  it  was,  powerfully  brought  out  by  the  mercury  vapour ;  but  on  all  those 
parts  to  which  the  iodine  vapour  has  had  access  the  mercury  will  not  adhere,  but  the 
phenomenon  will  take  effect  as  though  the  plate  had  never  been  exposed  to  the  light, 
except  on  those  portions  on  which  the  object,  whose  spectral  image  appears,  was  laid. 

716.  From  this  it  would  seem  that  the  vapour  of  iodine  has  the  quality  of  detithon- 
izing  a  surface  that  has  been  changed  by  light. 

717.  The  same  process  may  be  conducted  so  as  to  give  a  still  more  striking  result. 

718.  Employing  a  prepared  bromo-iodized  plate,  as  before,  expose  it  to  any  uniform 
source  of  light  for  such  a  length  of  time  that,  if  it  were  mercurialized,  it  would  whiten 
uniformly,  and  exhibit  the  aspect  of  an  ordinary  white  Daguerreotype.  Treat  it  as 
before,  by  placing  on  it  any  object,  pass  it  into  the  vapour  of  iodine,  remove  the  object, 
and  mercurialize ;  and  now  a  spectral  appearance  of  that  object,  of  a  dense  whife  as- 
pect, will  emerge,  the  remainder  of  the  plate  being  quite  black  and  in  the  condition 
of  the  shadows  of  a  Daguerreotype,  that  is,  as  though  it  had  never  been  exposed  to 
the  light. 

719.  In  order  to  obtain  a  clear  idea  of  what  passes  under  the  foregoing  circumstances, 
I  made  the  following  O'ial. 

Upon  a  plate  prepared  and  deeply  tithonized,  as  has  been  said,  I  laid  a  double 
convex  lens  of  about  two  inches  focus,  and  exposing  the  plate,  with  the  lens  upon  it, 
to  the  vapour  of  iodine,  and  then  removing  the  lens,  I  mercurialized.  A  deep-blue 
spectral  image  emerged,  of  less  diameter  than  the  lens,  but,  like  it,  of  a  circular  form,  its 
circumference  being  marked  by  as  sharp  a  line  as  if  it  had  been  drawn  by  a  pair  of 
compasses.  Indeed,  it  looked  as  distinct  and  as  sharj)  as  if  a  blue  wafer  had  been  laid 
on  the  plate. 

720.  In  several  successive  trials  I  found  that  tne  magnitude  of  this  spectre  dimin- 
ished as  the  time  of  exposure  to  the  vapour  had  been  prolonged. 

721.  Next,  I  repeated  the  same  trial,  using  the  plate  and  lens  as  just  described  ; 
hut  immersing  the  plate  in  the  vapour  of  bromine  instead  of  that  of  iodine,  a  still 


DETITHONIZING  ACTION. 


167 


more  remarkable  image  emerged  on  mercurializing.  This  image,  like  the  former,  was 
circular  and  black,  but  all  around  it  for  a  certain  space  there  was  an  annulus  of  nar- 
row dimensions  of  pure  unmercuriallzed  silver,  the  deep  black  of  which  contrasted 
strikingly  with  the  blue  black  of  the  spectre,  and  its  outer  circumference  was  marked 
by  a  faint  whitening  of  the  plate — faint,  but  as  sharp  as  it  is  possible  to  conceive. 

722.  In  a  third  trial,  things  were  conducted  as  before,  except  that  now  chlorine, 
diluted  with  atmospheric  air,  was  used  ;  the  spectre  again  came  out,  and  did  not  differ 
m  any  observable  manner  from  that  produced  by  iodine. 

723.  In  a  fourth  trial,  the  vapour  of  nitrous  acid  was  used  as  a  detithonizer.  In 
this  case,  the  edges  of  the  spectre  commonly  had  a  gradually  shading  outline,  and  only 
in  one  instance  did  I  find  that  sharpness  of  termination  exhibited  in  the  other  cases. 

724.  We  therefore  perceive  that  iodine,  bromine,  chlorine,  and  nitrous  acid  can  de- 
tithonize  a  surface  on  which  hght  has  fallen :  they  can  undo  what  the  tithonic  rays  have 
done. 

725.  In  repeating  these  experiments,  as,  for  example,  the  one  by  iodine,  if  the  com- 
mon iodine-box  be  used  to  effect  the  detithonization,  two  or  three  seconds  of  time  is 
all  that  is  required.  A  longer  period  is  demanded  when  the  vapour  is  very  weak,  but 
when  strong  the  effect  is  almost  instantaneous. 

726.  This  detithonization  and  production  of  spectral  images  can  therefore  be  ac- 
complished in  an  incredibly  short  space  of  time. 

727.  I  made  trials  with  other  substances,  such  as  hydrogen  gas  and  the  vapour  of 
liquid  muriatic  acid.  The  former  to  a  certain  extent,  though  not  near  so  powerfully 
as  the  electro-negative  bodies  mentioned,  could  produce  the  change  in  question  ;  the 
latter  seemed  to  be  without  any  perceptible  action. 

728.  To  the  list,  with  the  other  electro-negative  substances,  I  believe  oxygen  ought 
to  be  added  ;  for,  on  repeating  the  same  experiment,  and  raising  the  temperature  of  the 
plate  in  atmospheric  air  so  as  to  maintain  the  tithonized  surface  at  about  200'  Fah.  for 
several  minutes,  a  certain  effect,  which  in  an  imperfect  way  resembled  those  already 
described,  was  exhibited.  Oxygen,  therefore,  diluted  as  in  atmospheric  air,  at  200°  Fah., 
ma}  be  regarded  as  possessing,  to  a  small  extent,  the  property  in  question. 

729.  Without  multiplying  the  description  of  these  experiments  farther — for  the  in- 
genuity of  any  one  who  repeats  them  will  suggest  many  modifications  which  may  give 
rise  to  striking  results — I  will,  in  conclusion,  give  the  reasons  which  have  led  me  to 
suppose  that  in  all  these  phenomena  two  different  principles  are  engaged — vapour  ac- 
tion and  radiation. 

730.  I  have  stated  that  the  electro-negative  bodies  possess  this  detithonizing 
quahty  in  a  very  marked  manner.  I  do  not  wish  it  to  be  understood,  however,  that 
there  is  any  relation  of  antagonization  between  that  particular  class  of  substances  and 
the  tithonic  rays.  ^It  appears  to  me  that  their  peculiar  quahty,  in  the  circumstances 
described,  may  be  traced  to  the  fact  that  silver,  an  electro-positive  substance,  happens 
to  afford  the  sensitive  surface.  I  have,  however,  prepared  a  paper  which  takes  up  the 
consideration  of  the  conditions  and  theory  involved,  and  will  not  at  present  anticipate 
what  has  to  be  offered  when  that  paper  shall  be  published. 


168 


DETITHONIZING  ACTION. 


731.  The  action,  then,  which  these  different  gases  and  vapours  exhibit  is  so  intense 
as  to  mask  the  feebler  effect  of  radiation.  Thus,  it  takes  several  hours'  exposure 
in  the  dark,  and  after  a  long  subsequent  process  of  mercurialization,  to  prove  the  true 
radiant  effect,  a  slow  detithonization,  which  could  be  brought  about  by  vapour  action 
in  an  instant.  But  whoever  has  seen  the  symmetrical,  or,  rather,  geometrical  lines 
that  are  left  when  the  slower  process  is  followed,  must  be  struck  with  the  persuasion 
that  the  phenomena  he  witnesses  is  obeying  geometrical  laws,  and  is  not  due  to  the 
irregular  action  of  a  dilute  and  varying  current  of  vapour. 

732.  Thus,  on  repeating  carefully  the  experiment  cited  at  the  close  of  my  last  paper, 
in  which  a  lens  is  laid  on  a  tithonized  surface  and  left  in  the  dark,  I  found  that  after 
the  mercurial  process  was  completed,  the  plate  exhibited  a  dark  central  spot  sur- 
rounded by  a  white  annulus.  On  drawing  upon  paper  a  section  of  the  lens  and  the 
sensitive  surface,  I  found  that  a  line  drawn  from  the  extreme  edge  of  the  white  annu- 
lus to  the  edge  of  the  lens  was  a  tangent  to  the  lens  at  that  point ;  that  a  line  drawn 
from  the  extreme  edge  of  the  central  dark  spot  would,  after  reflexion  by  the  convex 
surface  of  the  lens,  be  found  precisely  on  the  edge  of  the  white  annulus ;  the  edge  of 
the  annulus  and  the  edge  of  the  spot  thus  having  a  true  catoptrical  relation  to  the  curva- 
ture of  the  lens. 

733.  Now,  although  in  laboratories  such  as  that  in  which  my  experiments  are  con- 
ducted, the  vapours  of  these  different  electro-negative  bodies  are  unquestionably  pres- 
ent, and  may  produce  a  part  of  the  phenomena  witnessed,  yet,  inasmuch  as  that  phe- 
nomenon follows  laws  that  are  apparently  of  a  strict  geometrical  kind,  and  to  those 
floating  vapours  we  could  hardly  assign  anything  like  symmetrical  results — guided, 
also,  by  the  analogy  of  cooling  bodies,  which  lose  part  of  their  heat  through  radiation, 
and  part  through  the  current  action  of  the  air,  and  part  through  the  conducting  power 
of  their  supports,  I  have  been  led  to  take  the  view  of  the  phenomena  in  question 
which  I  have  set  forth. 


MODE  OF  PRODUCING  THE  FIXED  LINES. 


1C9 


CHAPTER  XIV. 

ON  A  NEW    SYSTEM    OF  INACTIVE   TITHONOGRAPHIC    SPACES    IN  THE   SOLAR  SPFCTRUM 
ANALOGOUS   TO   THE    FIXED   LINES   OF   FRAUNHOFER,  AND    ON   THE  TITHONOTYPE. 

{From  the  London,  Edinburgh,  and  Dublin  Philosophical  Magazine  for  May,  1843.) 

Contents  :  Mode  of  2^>'oducing  the  fixed  Lines. — Description  of  them. — Difficulty  of 
obtaining  them  in  the  Yellow  and  Green. 

Daguerreotypes  are  dotted  Surfaces. — Mode  of  copying  them  hy  the  Tithonotype. — Po- 
larized Structure  of  the  Daguerreotype  Film. 

734.  When  a  beam  of  the  sun's  light,  directed  horizontally  by  a  heliostat,  is  thrown 
into  a  dark  room,  and,  passing  through  a  chink  with  parallel  sides,  is  received  on  the 
surface  of  a  homogeneous  flint-glass  prism,  which  refracts  it  at  the  angle  of  minimum 
deviation,  and,  after  its  passage  through  the  prism,  is  converged  to  a  focal  image  on  a 
white  screen  by  the  action  of  an  achromatic  lens,  the  spectrum  which  results  is  given 
in  great  purity,  and  Fraunhofeu's  lines  are  quite  apparent.  The  larger  ones  are  seen 
by  the  most  casual  inspection. 

735.  A  tithonographic  surface,  after  being  placed  in  this  spectrum,  exhibits  impres- 
sions of  an  analogous  character,  being  covered  with  the  representations  of  multitudes 
of  inactive  lines  varying  greatly  in  dimensions. 

736.  After  several  attempts  last  summer,  I  succeeded  in  discovering  these  lines,  and 
have  obtained  impressions  of  them  sufficiently  perfect. 

737.  Before  proceeding  to  the  description  of  the  mode  which  is  to  be  followed,  and 
of  the  characters  of  the  lines  themselves,  I  cannot  avoid  calling  attention  to  the  re- 
markable circumstance,  which  has  frequently  presented  itself  to  me,  of  a  great  change  in 
the  relative  visibility  of  Fraunhofer's  lines,  when  seen  at  different  periods.  There  are 
times  at  which  the  strong  lines  seen  in  a  red  ray  are  so  feeble  that  the  eye  can  barely 
catch  them,  and  then,  again,  they  come  out  as  dark  as  though  marked  in  India  ink  on  the 
paper.  During  these  changes,  the  other  lines  may  or  may  not  undergo  corresponding 
variations.  The  same  observation  equally  applies  to  the  blue  and  yellow  rays.  It  has 
seemed  to  me  that  the  lines  in  the  red  are  more  visible  as  the  sun  approaches  the  hori- 
zon, and  those  at  the  more  refrangible  end  of  the  spectrum  are  obvious  in  the  middle  of 
the  day. 

738.  A  beam  of  the  sun,  passing  horizontally  from  a  heliostat  mirror  into  a  dark 
room,  was  received  on  a  screen  with  a  slit  in  its  centre,  the  slit  being  formed  by  a  pair 
of  parallel  knife  edges,  one  of  which  was  movable  by  a  micrometer  screw ;  the  instru- 
ment being,  in  fact,  the  common  instrument  used  for  showing  difli-acted  fringes.  The 
screw  was  adjusted  so  as  to  give  an  aperture  jV  inch  wide,  and  the  light,  passing  through, 
fell  upon  an  equiangular  flint-glass  prism  placed  at  the  distance  of  eleven  feet.  Imme- 
diately on  the  posterior  face  of  the  prism,  the  ray  was  received  on  an  achromatic  lens, 


170 


DESCRIPTION  OF  THE  FIXED  LINES. 


the  object-g;lass  of  a  telescope,  and  brought  to  a  focus  at  the  distance  of  six  feet  six 
inches,  at  which  place  an  arrangement  was  adjusted  for  exposing  white  paper  screens, 
on  which  the  more  prominent  fixed  lines  might  be  seen,  and  their  position  marked,  or 
sensitive  plates  substituted  for  the  screens,  occupying  precisely  the  same  position.  The 
lines  on  the  screens  could,  therefore,  be  compared  with  those  on  the  sensitive  surfaces, 
as  to  position  and  magnitude,  with  considerable  accuracy.  In  these  trials  I  have  gen- 
erally used  an  achromatic  lens,  but  the  lines  can  be  beautifully  seen  by  employing  a 
common  double  convex,  if  the  screen  be  inclined  forward  in  the  way  described  in  (GG3), 
Either  way  answers  very  well. 

739.  In  order  to  identify  these  lines,  I  have  made  use  of  the  map  of  the  spectrum, 
puljlished  by  Professor  Powell,  in  the  Report  of  the  British  Association  for  1839. 
With  the  instrumental  arrangement  described  they  are  .exceedingly  distinct,  and  no  dif- 
ficulty arises  in  the  identification  of  the  more  prominent  ones.  The  spectrum  with 
which  I  have  worked  occupies  upon  the  screen  a  space  of  nearly  four  inches  and  a 
quarter  in  length  from  the  red  to  the  violet,  or,  more  correctly  speaking,  from  the  ray 
marked  in  that  map  A  to  the  one  marked  k.  In  stating,  however,  that  no  difficulty 
arises  in  identifying  these  lines,  I  ought  to  add  that  I  am  referring  to  that  particular 
map.  In  the  figure  annexed  to  Sir  John  Herschel's  Treatise  on  Light,  in  the  Ency- 
clopedia Metropolilana,  the  ray  marked  G  seems  to  differ  from  that  of  the  report.  But 
Professor  Powell's  map  being  drawn  from  his  personal  observations,  and  with  refer- 
ence to  these  very  difficulties,  as  it  coincides  with  my  own  observations  and  measures, 
I  have  employed  it,  and,  therefore,  take  the  letters  he  gives. 

740.  It  will  be  understood  that  the  whole  spectrum  and  all  its  lines  cannot  be  obtained 
at  one  impression.  The  difficulty  which  is  in  the  way  of  effecting  this  rests  in  the 
circumstance  that  different  regions  of  the  spectrum  act  with  different  power  in  pro- 
ducing the  proper  effect.  Thus,  if  on  common  yellow  iodide  of  silver  the  attempt  were 
made  to  procure  all  the  lines  at  one  trial,  it  would  be  found  that  the  blue  region  would 
have  passed  to  a  state  of  high  solarization,  and  all  its  fine  lines  become  extinguished 
by  being  overdone  long  before  any  well-marked  action  could  be  traced  at  the  less  re- 
frangible extremity.  We  have,  therefore,  to  examine  the  different  regions  in  succes- 
sion, exposing  the  sensitive  surface  to  each  for  a  suitable  length  of  time. 

741.  \vi.  fig.  103,  I  have  given  on  the  left  side  a  representation  of  the  larger  lines  of 
Fraunhofer,  the  letters  l)eing  derived,  as  has  been  said,  from  Professor  Powell's  map. 
The  position  of  the  lines  is,  however,  copied  from  my  own  spectrum  as  closely  as  I 
have  been  able  to  accomplish  it. 

742.  In  order  that  a  comparison  may  be  made  between  the  new  system  of  lines  and 
those  of  Fraunhofer,  the  right  side  of  the  plate  gives  a  tithonographic  representation 
of  them  as  obtained  on  a  Daguerreotype  plate  which  has  been  iodized  to  the  yellow, 
brought  by  the  vapour  of  bromine  to  the  red,  and  then  slightly  exposed  to  the  vapour 
of  chloride  of  iodine.  The  map  is  so  adjusted  in  the  plate  as  to  have  its  lines  by  the  side 
of  those  of  Fraunuoffr  which  have  the  same  name.  Referring,  therefore,  to  the  plate,  it 
w  ill  be  seen  that  there  are,  beyond  the  red  ray,  three  extra-spectral  lines,  which  I  have 
marked  «,  [i,  y.    These,  however,  I  have  only  occasionally  found,  for,  from  the  general 


DIFFICULTY  OF  OBTAINING  THEM  IN  THE  YELLOW  AND  GREEN.  271 

diminution  of  effect  in  that  region,  they  do  not  always  come  out  in  a  plain  and  striking 
manner.  None  of  Fraunhofer's  lines  in  the  yellow  and  green  are  given,  but  G  and  its 
companions  are  very  strongly  marked,  as  also  the  group  about  i.  Bat  by  far  the  most 
striking  in  the  whole  tithonograph  are  those  marked  H  and  k  ;  and  now,  passing  beyond 
the  violet,  and  out  of  the  visible  limits  of  the  spectrum,  four  very  striking  groups  make 
their  appearance.  The  first  line  of  each  of  these  groups  I  have  marked,  in  continuation 
of  Fraunhofer's  nomenclature,  M,  N,  O,  P.  In  L  there  are  three  hnes,  in  M  five,  in 
N  three,  in  O  three,  and  in  P  five. 

743.  Besides  these  larger  groups,  the  whole  tithonograph  is  crossed  by  hundreds  of 
minuter  ones,  so  that  it  is  utterly  impossible  to  count  them.  If,  as  it  has  been  said, 
nearly  000  have  been  counted  between  A  and  H,  I  should  think  there  must  be  quite 
as  many  between  H  and  P.  In  speaking,  therefore,  of  these  lines  as  though  they  were 
strong  individual  ones,  the  expression  is  to  be  taken  with  some  limitation.  It  is  quite 
likely  that  each  of  those  bolder  lines  is  made  up  of  a  great  number  that  are  excessively 
narrow  and  close  together. 

744.  If  the  absorptive  action  of  the  sun's  atmosphere  be  the  cause  of  this  phenome- 
non, that  action  takes  place  much  more  powerfully  on  the  more  refrangible  and  extra- 
spectral  region.  The  lines  exhibited  there  are  bold  and  strongly  developed ;  they  are 
crowded  in  groups  together. 

745.  I  cannot  doubt,  judging  from  analogy,  that,  by  proper  modes  of  investigation, 
similar  lines  might  be  detected  in  the  calorific  extra-spectral  region. 

746.  The  contrast  between  the  visible  and  titlionographic  spectra  is  maintained  by 
the  non-appearance  of  lines  in  the  yellow  and  green  regions.  Once  only  I  thought  I 
perceived  a  line  corresponding  to  Fraunhofer's  F,  but  it  was  exceedingly  faint,  and, 
on  the  whole,  doubtful.  < 

747.  Fraunhofer's  lines,  which  occur  on  the  orange,  yellow,  and  green  spaces,  thus 
leaving  no  corresponding  impression,  another  argument  is  furnished  of  the  independence 
of  the  tithonic  and  luminous  rays.  It  is  probable  that  more  perfect  arrangements  than 
I  have  used  would  give  the  whole  spectrum  as  though  it  were  full  of  these  inactive 
spaces  ;  and  in  stating  that  nothing  like  Fraunhofer's  lines  exist  in  those  medial  re- 
gions, I  therefore  simply  wish  it  to  be  understood  that  I  can  find  nothing  at  all  cor- 
responding in  magnitude  to  the  great  lines  marked  D,  E,  F,  though  hundreds  of  micro- 
scopic ones  may  probably  exist  in  these  very  spaces. 

748.  The  position  of  the  lines  as  represented  on  the  sensitive  surface  is  found  to  be, 
as  might  have  been  anticipated,  independent  of  the  chemical  nature  of  that  surface. 
The  iodide  of  silver  gives  them  in  the  same  places  as  the  bromide. 

749.  An  argument  might  be  drawn,  as  has  been  said,  from  the  absence  of  these  lines 
in  the  yellow  and  green  spaces,  as  to  the  independence  of  the  dark  rays  and  light. 
This  is,  however,  only  another  proof  of  a  fact  of  which  we  have  now  abundant  evidence. 
In  1834,  when  my  attention  was  first  fixed  forcibly  on  these  things,  and  I  began  to 
make  prismatic  analyses  by  the  aid  of  sensitive  paper,  some  of  my  first  trials  were  di- 
rected to  the  detection  of  these  fixed  lines.  At  that  time  I  was  employing  sensitive 
paper  made  with  the  bromide  of  silver,  precisely  as  has  been  subsequently  done  in  Eu- 


172 


DAGUERREOTYPES  ARE  DOTTED  SURFACES. 


rope ;  a  number  of  the  results  were  published  in  the  American  Journals  during  the 
year  1837.  In  the  detection  of  these  lines  I  failed  entirely;  but  the  bromuretted  pa- 
per enabled  me,  at  that  early  period  (while  the  attention  of  no  other  chemist  was  as 
yet  turned  to  these  matters),  to  trace  the  blackening  action  from  far  beyond  the 
confines  of  the  violet  down  almost  to  the  other  end  of  the  spectrum.  I  distinctly 
made  out  that  the  dark  rays  underwent  interference  after  the  manner  of  their  luminous 
companions,  a  result  originally  due  to  Arago,  and  printed  some  long  papers  in  proof 
of  the  physical  independence  of  the  chemical  rays,  and  light,  and  heat,  throughout  the 
spectrum. 

750.  In  a  paper  "  On  the  Action  of  the  Rays  of  the  Solar  Spectrum  on  the  Da- 
guerreotype Plate,"  inserted  in  the  Philosophical  Magazine  for  the  month  of  February, 
1843,  which  has  just  reached  me,  Sir  John  Hekschel  points  out  that  a  connexion 
may  be  traced  between  the  phenomena  of  coloration  impressed  by  the  spectrum  and 
those  of  Newton's  rings.  With  striking  ingenuity,  he  shows  how  a  succession  of  pos- 
itive and  negative  pictures  may  arise  by  prolonged  solar  action,  and  those  shades  of 
colour  which  the  iodide  of  silver  exhibits,  under  variable  exposure  to  light,  originate. 

751.  This  hypothesis,  however,  as  that  able  philosopher  proceeds  to  state,  is  not 
unattended  with  difficulties,  and  after  pointing  out  what  those  difficulties  are,  he  shows 
how,  nevertheless,  it  can  account  for  an  extensive  group  of  facts.  I  regret  that  these 
difficulties  are  in  the  way,  and  that  there  are  also  other  facts  which  appear  to  ex- 
clude the  theory  of  thin  plates  from  these  phenomena. 

752.  The  Daguerreotype  image,  in  all  its  forms,  may  he  transferred  by  any  cojrying 
process  to  other  suitable  surfaces.    In  other  words,  it  may  he  printed  from. 

753.  Sir  David  Brewster  was  the  first  to  show  that  the  colours  of  mother-of-pearl 
might  be  impressed  on  any  yielding  surface.  In  the  same  manner  so  can  the  Daguerre- 
otype image. 

754.  This  is,  unquestionably,  the  most  important  fact  yet  known  in  the  history  of 
these  mysterious  images,  both  in  a  theoretical  and  in  a  practical  point  of  view.  In 
a  theoretical  point  of  view,  it  shows  us  that  it  is  among  the  phenomena  of  grooved,  or 
striated,  or  dotted  surfaces  that  the  Daguerreotype  is  to  be  ranged ;  and,  in  a  practical 
point  of  view,  it  shows  the  true  mode  of  solving  the  great  problem  of  producing  from 
a  given  proof  a  multitude  of  copies. 

755.  In  (594),  in  speaking  of  the  action  of  isinglass  dried  on  the  surface  of  the 
Daguerreotype  pictures,  I  stated  that  I  had  succeeded  with  a  process  for  multiplying 
copies,  and  promised  on  a  future  occasion  to  make  it  known  :  that  promise  I  now  pro- 
ceed to  redeem. 

756.  On  referring  to  the  paper  in  question,  the  reader  will  perceive  that  the  follow- 
ing facts  are  stated  (577)  :  that  gum-arabic  mucilage,  dried  on  a  common  Daguerre- 
otype, splits  up,  bringing  with  it  the  white  portions;  that  Russian  isinglass,  (584)  and 
(591),  dried  in  a  similar  manner,  does  th  ^  same  thing,  and  will  even  rend  off  the  yel- 
low coating  of  iodine  if  it  has  not  be  n  previously  removed. 

757.  Now,  in  addition,  I  have  to  state  that,  if  on  a  picture  that  has  been  fixed  by  a 
film  of  gold,  so  as  to  be  irremovable,  a  lay^  r  of  isinglass  be  caused  to  dry  and  split  up, 


MODE  OF  COPYING  THEM  BY  THE  TITHONOTYPE.  273 

it  will  bear  on  its  surface  a  complete  impression  of  the  drawing,  all  the  details  being 
given  with  inexpressible  beauty,  the  minutest  lines  and  dots  being  present. 

758.  From  the  same  plate  a  series  of  these  impressions  may  be  taken.  The  images 
that  are  on  them  may  be  seen  either  by  reflected  or  transmitted  light ;  in  the  former  in- 
stance, most  favourably  by  placing  them  on  black  velvet. 

759.  I  have  hopes  of  improving  this  method  so  as  to  introduce  it  into  efifectual  use. 
The  practical  difficulties  that  are  in  the  w^ay  rest  in  the  circumstance  that  the  isinglass 
often  splits  off  in  chips  instead  of  separating  in  one  unbroken  sheet.  And  the  plate 
from  which  the  impressions  are  taken,  or  with  which  the  printing  process  is  carrying 
on,  becomes  injured ;  not  by  having  its  surface  removed,  but  by  the  isinglass  adhering 
in  circumscribed  places,  and  obstinately  refusing  to  detach  itself. 

760.  This  refinement  on  the  art  of  printing,  or,  rather,  of  casting,  might  be  supposed 
to  give  rise  to  very  perishable  results.  This,  however,  is  far  from  the  case  ;  I  have 
now  by  me  proofs  made  nearly  two  years  ago,  and  they  do  not  seem  to  have  under- 
gone any  change.    They  have  lain  loosely  in  a  drawer. 

761.  I  presume,  therefore,  that  any  process  which  can  exhibit  the  colours  of  mother- 
of-pearl  will  also  exhibit  Daguerreotype  images.  This  lays  open  a  variety  of  new 
branches  of  the  photographic  art. 

762.  As  a  name  for  these  processes  of  copying  the  surface  of  a  Daguerreotype,  I 
would  suggest  the  word  Tithonotype. 

763.  To  carry  this  process  into  effect,  the  operator  proceeds  as  follows:  The  Da- 
guerreotype which  he  designs  to  copy  is  to  be  covered  with  a  thin  film  of  gold  in  the 
usual  way,  care  being  taken  that  the  film  is  neither  too  thick  nor  too  thin.  If  it  be  too 
thick,  the  resulting  copy  is  injured,  and  difficulties  are  more  liable  to  arise  in  effecting 
the  separation  of  the  gelatinous  coat;  if  too  thin,  the  plate  itself  will  suffer  injury  by 
having  the  figure  torn  off. 

764.  A  clear  solution  of  isinglass  is  next  to  be  prepared  ;  it  must  be  of  such  a  con- 
sistency that  a  drop  of  it  poured  on  a  cold  metallic  plate  will  speedily  set.  Much  of 
the  success  of  the  process  depends  on  this  solution  being  properly  made.  There  is  a 
siibstance  in  the  market  which  goes  under  the  name  of  Cooper's  isinglass,  which  I  have 
found  much  better  than  any  other  for  these  purposes. 

765.  The  plate  is  to  be  arranged  horizontally,  with  its  face  upward,  on  some  proper 
support,  in  the  current  of  hot  air  that  rises  from  a  stove.  The  isinglass  is  to  be  poured 
on  until  a  stratum  about  '  of  an  inch  deep  is  upon  the  plate.  It  is  then  suffered  to 
dry,  the  process  being  conducted  so  as  to  occupy  two  or  three  hours.  When  perfectly 
successful,  as  soon  as  the  drying  is  complete,  the  film  of  isinglass,  now  indurated  into  a 
tithonotype,  splits  off,  and  on  being  examined  either  by  reflected  or  transmitted  light,  will 
be  found  to  bear  a  minute  copy  of  the  original. 

766.  To  return  for  a  while  to  the  theory  of  these  images.  While  thus  it  is  plain 
that  the  optical  effect  depends  on  surface  configuration  alone,  and  does  not  seem  to 
have  any  immediate  relation  to  the  thickness  or  thinness  of  a  film,  it  is  very  different 
with  the  chemical  effect  on  which  the  whole  phenomenon  depends. 

767.  Tlie  Daguerreotype  film,  which  lias  been  under  the  irrfluence  of  light,  is  po- 
larized throughout  its  structure  previous  to  mercurialization. 


174 


DARK  TITHONIC  RAYS. 


768.  I  use  the  word  "  polarized"  in  its  chemical  sense.  An  illustration  will  serve 
to  show  the  signification  I  attach  to  the  term.  When  water  is  placed  between  platina 
electrodes,  its  oxygen  is  liberated  from  one  of  them,  and  its  hydrogen  from  the  other, 
and  the  intervening  liquid  assumes  a  polar  state,  a  series  of  decompositions  and  combi- 
nations going  on.  As  that  water  is  polarized,  and  undergoes  polar  decomposition,  so, 
too,  do  the  same  phenomena  hold  in  the  case  of  the  Daguerreotype  film. 

1st.  We  know  that  no  iodine  is  ever  evolved  from  the  plate,  even  under  the  most 
prolonged  action  of  the  light  (587). 

2d.  The  cause  of  the  final  appearance  of  the  image  is  due  to  silver  being  liberated  on 
the  anterior  face  of  the  plate  (592). 

3d.  When,  by  the  action  of  gelatine,  the  iodine  and  mercury  are  both  removed  from 
the  plate,  it  is  obvious  that  the  plate  has  been  corroded  wherever  the  light  fell.  Iodine, 
therefore,  has  been  evolved  on  the  posterior  face  of  the  film,  and  is  the  cause  of  this  cor- 
rosion. 

769.  From  the  circumstance,  therefore,  that  iodine  is  evolved  at  the  back  of  the  film 
and  silver  at  its  front,  and  the  film  itself  remaining  the  same  in  thickness  throughout,  it 
is  obvious  that  there  is  a  strong  resemblance  between  this  phenomena  and  that  of  the 
polar  decomposition  of  water.  The  electro-positive  and  electro-negative  elements  are 
yielded  up  on  opposite  faces  of  the  film,  and  its  interior  undergoes  incessant  polar  chan- 
ges, the  opposite  electric  particles  sliding,  as  it  were,  on  one  another. 

770.  In  a  note  to  the  preceding  chapter  (708,  &c.)  I  have  described  the  remarkable 
power  of  certain  electro-negative  gases  in  producing  the  rapid  detithonization  of  sur- 
faces that  have  been  changed  by  light.  Since  that  paper  was  sent  to  England,  I  per- 
ceive, from  the  "  Scientific  Memoirs,"  that  Professor  Moser  has  published  results  of  a 
similar  kind.  The  true  explanation  of  them  appears  to  me  to  be  very  different  from 
that  which  he  gives ;  for  his  idea  of  vapours  containing  latent  rays  of  particular  orders  of 
refrangil)ility  or  colour,  rests  on  a  very  feeble  analogy,  and  strikes  me  as  entirely  without 
support. 

771.  The  view  which  I  have  taken  of  these  phenomena,  and  to  which  allusion  was 
made  in  the  paper  referred  to,  can  be  easily  understood  from  what  has  just  been  said. 
The  film  on  a  Daguerreotype  plate,  which  has  been  disturbed  by  the  tithonic  rays,  but 
not  yet  mercurialized,  is  in  a  polar  condition  of  force,  its  iodine  is  ready  to  unite  with 
a  new  layer  of  silver  behind,  its  silver  is  ready  to  be  evolved  in  front.  If  it  be  expo- 
sed to  mercurial  vapours,  union  at  once  takes  place  on  that  front  face,  and  an  amalgam 
is  formed  ;  if  to  the  vapours  of  iodine,  or  chlorine,  or  bromine,  an  iodide,  chloride,  or 
bromide  of  silver  is  formed.  In  an  instant,  its  disturbing  affinities  being  satisfied,  the 
film  reverts  back  to  its  former  condition  of  equilibrium,  and  is  precisely  in  the  condition 
it  was  in  before  exposure  to  the  light. 


DR.  DAUBENY'S  EXPERIMENTS,  275 


CHAPTER  XV. 

ON  THE   DECOMPOSITION  OF   CARBONIC  ACID   GAS  AND   THE  ALKALINE   CARBONATES  BY 
THE  LIGHT  OF  THE  SUN,  AND  ON  THE  TITHONOTYPE. 

(From  the  London,  Edinburgh,  and  Dublin  Philosophical  Magazine  for  September,  1843.) 

Contents  :  Dr.  Dauheny's  Experiments. — Importance  of  the  Subject. — Decomposition 
in  the  Prisinatic  Spectrum. — Decomposition  under  Absorbent  Media. — Decompositio7i 
is  due  to  Light.  —  Disturbing  Causes. — Analysis  of  Gas  evolved. —  Decomposition  of 
Saline  Bodies. — Production  of  Nitrogen. — Disappearance  of  Oxygen. — Character  of 
Chlorophyll. 

Tithonotypes  in  Copper. — Detithonizing  Poicer  of  Gases. 

772.  For  many  years  it  has  been  known  that  the  green  parts  of  plants  under  the 
influence  of  the  sunhght  possess  the  power  of  decomposing  carbonic  acid,  and  setting 
free  its  oxygen.  It  is  remarkable  that  this,  which  is  a  fundamental  fact  in  vegetal)le 
physiology,  should  not  have  been  investigated  in  an  accurate  manner.  The  statements 
met  with  in  the  books  are  often  far  from  being  correct.  It  is  sometimes  said  that  pure 
oxygen  gas  is  evolved,  that  the  decomposition  is  brought  about  by  the  so-called  "chem- 
ical rays  ;"  these,  and  a  multitude  of  other  such  errors,  pass  current.  So  far  as  my 
reading  goes,  no  one  has  yet  attempted  an  analysis  of  the  phenomenon  by  the  aid  of  the 
prism,  the  only  way  in  which  it  can  be  truly  discussed. 

773.  In  a  paper  by  Dr.  Daubeny,  inserted  in  the  Philosophical  Transactions  for  1836, 
two  facts,  which  I  shall  verify  in  this  communication,  are  fully  established.  These  are, 
1st,  the  constant  occurrence  of  nitrogen  gas  in  mixture  with  the  oxygen,  an  observation 
originally  due  to  Saussure,  or  some  earlier  writer ;  and,  2d,  that  the  act  of  decompo- 
sition is  due  to  the  light  of  the  sun.  This  latter  result,  obtained  by  employing  col- 
oured glasses  or  absorbent  media,  has  not  been  generally  received.  Doubt  will  always 
hang  about  results  obtained  in  this  way,  and  nothing  but  an  analysis  by  the  prism  can 
be  satisfactory.  It  has  happened,  therefore,  in  books  of  credit  pul)lished  since  that  time, 
that  other  interpretations  of  the  phenomena  have  been  given, — Johnsons  Agr.  Chem., 
Lect.  v.,  §  7 ;  Graham's  Chem.,  p.  1013, 

774.  In  its  connexions  with  modern  organic  chemistry  and  physiology,  the  experi- 
ment of  the  decomposition  of  carbonic  acid  by  leaves  assunies  extraordinary  interest. 
To  no  other  single  experiment  can  the  same  importance  be  attached.  When  we  re- 
member that  this  decomposition  is  the  starting-point  for  organization  out  of  dead  mat- 
ter, that,  commencing  with  this  action  of  the  leaf,  the  series  of  organized  atoms  goes 
forward  in  increasing  complexity,  and  blood,  and  flesh,  and  cerebral  matter  are  at  its 
terminus,  it  is  clear  that  unusual  importance  belongs  to  precise  views  of  this,  the  com- 
mencing change.    The  beams  of  the  sun  are  the  authors  of  all  organization. 

775.  There  is  but  one  way  by  which  the  question  can  be  finally  settled,  and  that  is 
by  conducting  the  experiment  in  the  prismatic  spectrum  itself    When  we  consider  the 


176 


IMPORTANCE  OF  THE  SUBJECT. 


feebleness  of  effect  which  takes  place  by  reason  of  the  dispersion  of  the  incident  beam 
through  the  action  of  the  prism,  and  the  great  loss  of  light  through  reflection  from  its 
surface,  it  might  appear  a  difficult  operation  to  effect  a  determination  in  this  way.  En- 
couraged, however,  by  the  purity  of  the  skies  in  America,  I  made  the  trial,  and  have  met 
with  complete  success. 

776.  Before  entering  on  the  experiments  which  I  have  to  communicate,  I  cannot 
avoid  once  more  impressively  calling  the  attention  of  chemists  to  the  true  character  of 
those  emanations  which  are  here  designated  "tithonic  rays."  It  is  not  enough  that  we 
admit  the  existence,  throughout  the  spectrum,  of  dark  rays,  possessing  the  power  of 
bringing  about  chemical  changes ;  it  is  not  enough  that  we  call  them  chemical  rays ; 
there  are  qualities  of  distinction  appertaining  to  them  which  mark  them  out  as  being 
specific  in  their  kind,  properties  which  they  possess  totally  distinct  from  those  of  light 
and  heat.  Their  title  to  the  rank  of  a  distinct  imponderable  agent  is  just  as  perfect  as 
that  of  Ught  or  heat.  From  heat  they  are  to  be  distinguished  by  incapacity  for  metallic 
conduction,  and  by  want  of  the  power  of  expanding  bodies ;  from  light,  by  failing  to 
give  any  impression  to  the  organ  of  vision.  According  to  the  recognised  rules  of  chem- 
istry, they  ought  to  be  received  as  a  fourth  imponderable  agent. 

777.  It  is  not  sufficient,  as  has  been  said,  to  call  them  "chemical  rays."  The  term 
implies  that  the  distinctive  characteristic  pertaining  to  them  is  the  power  of  changing 
the  composition  of  bodies.  But  do  not  the  rays  of  heat  eminently  produce  like  chan- 
ges 1  Are  not  half  the  decompositions  in  chemistry  brought  about  by  the  action  of  ca- 
loric ?  As  respects  light,  many  instances  are  already  known  in  which  it  produces  de- 
compositions and  combinations ;  as  will  be  presently  shown,  it  is  the  agent  that  brings 
about  the  decomposition  of  carbonic  acid.  The  faculty  of  producing  a  like  effect  is  not 
the  distinguishing  quality  of  the  tithonic  rays,  nor  can  the  term  chemical  be  any  more 
applied  to  them  than  to  either  of  their  acknowledged  distinct  companions.  Unless, 
titerefore,  chemists  are  content  to  admit  that  a  species  of  heat  may  exist  devoid  of  the 
power  of  expanding  bodies,  of  giving  the  sensation  of  warmth,  and  of  being  trans- 
mitted by  conducting  processes ;  or,  unless  they  admit  that  light  can  exist  in  such  a 
modified  condition  as  to  produce  in  our  eyes  the  sensation  of  darkness,  they  will  have  to 
admit  these  tithonic  rays  as  constituting  a  fourth  imponderahle  agent.  The  name  they 
may  take  is  not  a  matter  of  importance;  that  which  is  least  trammelled  by  hypoth- 
esis is  best.  It  is  not  the  object  of  this  and  the  foregoing  chapters  to  show  merely 
that  a  class  ot"  invisible  rays  exists  in  the  spectrum  ;  that  has  been  known  for  a  long 
time ;  but  it  is  to  point  out  the  true  relation  of  these  rays  to  other  bodies  and  other 
forces  in  the  world,  to  assert  for  them  their  title  of  a  fourth  distinct  imponderable  agent, 
and  to  secure  for  them  the  admission  of  that  title  by  giving  them  a  name. 

778.  When  the  leaves  of  plants  are  placed  in  water  from  which  all  air  has  been  ex- 
pelled by  boiling,  and  exposed  to  the  sun's  rays,  no  gas  whatever  is  evolved  from  them. 
When  they  are  placed  in  common  spring  or  pump  water,  bubbles  quickly  form,  which, 
when  collected  and  analyzed,  prove  to  be  a  mixture  of  oxygen  and  nitrogen  gases  ; 
from  a  given  quantity  of  water  a  fixed  quantity  of  air  is  produced.  When  they  are 
exposed  in  water  which  has  been  boiled  and  then  impregnated  with  carbonic  acid,  the 
decomposition  goes  on  with  rapidity,  and  large  quantities  of  gas  are  evolved. 


DECOMPOSITION  IN  THE  PRISMATIC  SPECTRUM. 


177 


779.  The  obvious  inference  which  seems  to  arise  from  tliese  facts  is,  that  all  the  ox- 
ygen collected  is  derived  from  the  direct  decomposition  of  carbonic  acid.  We  shall 
presently  examine  whether  this  is  the  correct  inference. 

780.  Having,  by  long  boiling  and  subsequent  cooling,  obtained  water  free  from  dis- 
solved air,  I  saturated  it  with  carbonic  acid  gas.  Some  grass  leaves,  the  surfaces  of 
which  were  carefully  freed  from  any  adhering  bubbles  or  films  of  air  by  having  been 
kept  beneath  carbonated  water  for  three  or  four  days,  were  provided.  Seven  glass 
tubes,  each  half  an  inch  in  diameter  and  six  inches  long,  were  filled  with  carbonated 
water,  and  into  the  upper  part  of  each  the  same  number  of  blades  of  grass  were 
placed,  care  being  taken  to  have  all  as  near  as  could  be  alike.  The  tubes  were  in- 
serted side  by  side  in  a  small  pneumatic  trough  of  porcelain.  It  is  to  be  particularly 
remarked  that  the  blades  were  of  a  pure  green  aspect,  as  seen  in  the  water;  no  glisten- 
ing air-film,  such  as  is  always  on  freshly-gathered  leaves,  nor  any  air  bubbles,  were 
attached  to  them.  Great  care  was  taken  to  secure  this  perfect  freedom  from  air  at  the 
outset  of  the  experiments. 

781.  The  little  trough  was  now  placed  in  such  a  position  that  a  solar  spectrum, 
kept  motionless  by  a  heliostat  and  dispersed  by  a  flint-glass  prism  in  a  horizontal  di- 
rection, fell  upon  the  tubes.  By  bringing  the  trough  nearer  to  the  prism  or  moving  it 
farther  off,  the  different  coloured  spaces  could  be  made  to  fall  at  pleasure  on  the  inverted 
tubes.  The  beam  of  light  was  about  three  fourths  of  an  inch  in  diameter.  In  a  few 
minutes  after  the  commencement  of  the  experiment,  the  tubes  on  which  the  orange, 
yellow,  and  green  light  fell,  commenced  giving  off  minute  gas  bubbles,  and  in  about  an 
hour  and  a  half  a  quantity  was  collected  sufficient  for  accurate  measurement. 

782.  The  gas,  thus  collected  in  each  tube,  having  been  transferred  to  another  ves- 
sel and  its  quantity  determined,  the  little  trough,  with  all  its  tubes,  was  freely  exposed 
to  the  sunshine.  All  the  tubes  now  commenced  actively  evolving  gas,  which,  when  col- 
lected and  measured,  served  to  show  the  capacity  of  each  tube  for  carrying  on  the  pro- 
cess. If  the  leaves  in  one  were  more  sluggish,  or  exposed  a  smaller  surface  than  the 
others,  the  quantity  of  gas  evolved  in  that  tube  was  correspondingly  less.  As  may  be 
readily  supposed,  I  never  could  get  tubes  so  arranged  as  to  act  precisely  alike,  but  after 
a  httle  practice  I  brought  them  sufficiently  near  to  equality.  And  in  no  instance  was 
this  testing  process  of  the  power  of  each  tube  for  evolving  gas  omitted  after  the  experi- 
ment in  the  spectrum  was  over. 

TABLE  OF  THE  DECOMPOSITION  OF  CARBONIC  ACID  BY  LIGHT  OF  DIFFERENT  COLOURS. 


Experiment  1. 

Experiment  2. 

Name  of  Ray. 

Volume  of  Gas. 

Name  of  Ray. 

Volume  of  Gas. 

Extreme  red   .  . 

■33 

Extreme  R.  and  red 

■00 

Red  and  orange  . 

20  00 

Red  and  orange  .  . 

24  75 

Yellow  and  green 

36  00 

Yellow  and  green  . 

43  75 

Green  and  blue 

•10 

Green  and  blue  .  . 

410 

Blue  

■00 

Blue  

100 

Indigo  .... 

■00 

Indigo  

■00 

Violet  .... 

■00 

Violet  

■00 

783.  From  this,  it  appears  that  the  rays  which  cause  the  decomposition  of  carbonic 
acid  gas  have  the  same  place  in  the  spectrum  as  the  orange,  the  yellow,  and  the 
green ;  the  extreme  red,  the  blue,  the  indigo,  and  the  violet  exerting  no  perceptible  ef- 


178 


DECOMPOSITION  UNDER  ABSORBENT  MEDIA. 


feet.  This  being  the  case,  we  should  expect  that,  by  passing  a  beam  through  absorb- 
ent media  of  such  a  nature  that  the  extreme  red,  the  bhie,  the  indigo,  and  violet  are 
absorbed,  this  decomposition  sliould  nevertheless  go  on.  A  solution  of  bichromate  of 
potash  nearly  fulfils  these  conditions,  and  not  only  does  it  absorb  the  luminous  rays  in 
question,  but  also  all  the  tithonic  rays,  except  a  trace  of  those  which  correspond  to  the 
more  refrangible  yellow  and  less  refrangible  green. 

784.  A  remarkable  proof  of  the  correctness  of  the  foregoing  prismatic  analysis 
comes  out  when  leaves  are  made  to  act  on  carbonated  water  in  light  which  has  passed 
through  a  solution  of  bichromate  of  potash.  I  took  a  wooden  box  of  about  a  cubic 
foot  in  dimensions,  and  having  removed  its  bottom,  adjusted  to  it  a  trough  made  of 
pieces  of  plate  glass.  The  box  being  set  on  end,  its  lid  served  as  a  door,  and  the 
trough  being  filled  with  a  solution  of  the  bichromate  of  potash,  the  sun's  beams  came 
through  it,  and  in  the  interior  of  the  box  an  arrangement  of  leaves  and  carbonated  wa- 
ter could  be  exposed  to  the  rays  that  had  escaped  absorption.  The  thickness  of  the 
liquid  stratum  was  about  half  an  inch.  I  had  several  such  boxes  made,  so  that  I  might 
compare  the  simultaneous  effect  of  light  which  had  undergone  absorption  by  different 
media.  They  formed,  as  it  were,  a  series  of  little  closets  in  which  bodies  could  be  ex- 
posed to  party-coloured  light — blue,  yellow,  red,  &c. 

785.  Whenever  an  experiment  was  commenced  in  these  closets,  simultaneously  a 
similar  one  was  commenced  in  the  unobstructed  sunshine.  It  is  needless  to  repeat, 
that  in  all  these  care  was  taken  to  have  the  different  arrangements  for  decomposition 
as  nearly  alike  as  possible. 

786.  On  comparing  together  the  amount  of  gas  evolved  in  unabsorbed  light  and  in 
light  that  had  undergone  absorption  by  the  bichromate  of  potash,  in  three  out  of  five 
trials  the  gas  collected  under  the  latter  circumstances  exceeded  in  volume  that  collected 
under  the  former ;  this  was  probably  due  to  a  slightly  higher  temperature  which  obtained 
in  the  box. 

787.  On  comparing  together  the  volumes  of  gas  collected  under  the  bichromate  of 
potash  and  under  litmus  water,  the  latter  was  not  equal  to  one  half  the  former. 

788.  I  compared  together  the  gas  evolved  in  unobstructed  light,  under  bichromate  of 
potash,  and  under  ammonio-sulphate  of  copper ;  the  results  were  as  follows : 

Unobstructed  light  ....  4"75 
Bichromate  of  potash  .  .  .  4-25 
Ammonio-sulphate  of  copper  .  •75 

789.  Comparing  these  experiments,  made  by  the  aid  of  absorptive  media,  with  those 
made  by  the  prism,  we  are  enabled  to  come  to  a  definite  conclusion  as  to  the  character 
of  the  rays  which  cause  this  decomposition. 

790.  The  true  office  of  prismatic  analysis  is  to  determine  the  refrangibility  of  the 
rays  which  produce  given  actions ;  but  inasmuch  as  rays  of  heat,  rays  of  light,  and 
tithonic  rays  are  found  throughout  the  spectrum,  in  many  cases  the  prism  fails  to  indi- 
cate to  which  of  these  imponderable  agents  phenomena  are  to  be  ascribed.  The  case 
before  us  furnishes  a  striking  example.  Although  the  decomposition  of  carbonic  acid 
is  most  energetically  brought  about  by  rays  whose  index  of  refraction  corresponds  to 


I 

DECOMPOSITION  IS  DUE  TO  LUaiT.  j^79 

the  yellow,  yet  that  region  of  the  spectrum  is  far  from  being  devoid  of  heat  and  tithon- 
icity. 

791.  By  considering,  however,  the  prismatic  analysis  and  the  absorptive  analysis  to- 
gether, the  following  facts  appear:  1st,  the  place  of  maxinuiin  action  in  the  spectrum 
corresponds  to  the  maximum  of  illumination  ;  2d,  at  the  place  of  the  maximum  of 
heat  (which  in  the  prism  here  used  is  beyond  the  extreme  red)  no  decomposition  what- 
ever takes  effect;  this  appears,  therefore,  to  exclude  calorific  influence;  3d,  the  point 
of  maximum  action  of  the  tithonic  rays,  which  escape  absorption  by  the  bichromate  of 
potash,  being  towards  the  green,  does  not  correspond  with  the  place  of  maximum  de- 
composition, which  is  the  yellow;  this  seems  to  exclude  the  tithonic  rays;  4th,  the  de- 
composition taking  place  almost  as  energetically  under  the  bichromate  of  potash  as  in 
the.  unobstructed  beams  of  the  sun,  and  that  salt  absorbing  all  but  a  mere  trace  of  the 
tithonic  rays,  if  the  effect  was  due  to  them  it  ought  to  be  retarded  to  an  extent  cor- 
responding to  their  loss  by  absorption,  which  is  far  from  being  the  case  ;  the  retardation 
which  is  observed  appearing  to  be  attributable  rather  to  the  loss  of  light  by  reflexion 
from  the  faces  of  the  trough,  and  the  partial  turbidity  (want  of  translucence)  of  its 
glasses  and  solutions. 

792.  For  these  reasons,  I  conclude  that  the  decomposition  of  carbonic  acid  by  the 
leaves  of  plants  is  brought  about  by  the  rays  of  light  ;  and  that  the  calorific  and 
tithonic  rays  do  not  participate  in  the  phenomenon.  As  was  stated  before,  therefore, 
the  rays  of  light  are  just  as  much  entitled  to  the  appellation  of  chemical  rays  as  those 
which  have  heretofore  passed  under  that  name. 

793.  I  might  observe,  in  passing,  that  there  is  a  degree  of  precision  attached  to 
results  of  the  decomposition  of  carbonic  acid  which  is  wholly  wanting  in  most  similar 
experiments.  In  the  stains  on  Daguerreotype  plates,  or  on  photographic  papers,  though 
there  is  no  difficulty  in  ascertaining  the  place  of  maximum  effect,  yet  nothing  in  the 
shape  of  absolute  measures  of  quantities  can  be  obtained.  When,  however,  gas  can 
be  collected  and  its  volume  determined,  as  in  the  voltameter  and  in  the  experiments 
just  described,  the  results  possess  a  degree  of  exactness  which  enables  us  to  draw  from 
them  definite  conclusions. 

794.  Let  us  now  proceed  to  determine  the  constitution  of  the  gaseous  mixture  given 
off  during  these  decompositions.  It  is  not  pure  oxygen,  as  has  often  been  supposed  and 
often  disproved,  but  a  mixture  of  oxygen,  nitrogen,  and  carbonic  acid.  It  is  luainly  to 
the  ratio  of  the  two  former  that  attention  has  to  be  directed ;  the  amount  of  the  latter  is 
always  variable  in  different  trials.  Before  proceeding  to  this,  there  are  certain  obser- 
vations to  be  premised,  the  results  of  which,  though  familiar  to  chemists  accustomed  to 
gaseous  analysis,  deserve  a  place  here,  for  they  seem  to  be  wholly  overlooked  in  many 
of  the  experiments  connected  with  the  so-called  respiration,  but,  rather,  digestion  of 
plants  recorded  in  the  books  of  botany. 

795.  When  gas  of  any  kind  is  confined  over  water  in  the  pneumatic  trough,  its  con- 
stitution is  undergoing  incessant  change.  A  portion  of  it  dissolves  more  or  less  slowly 
in  the  water,  and  in  exchange  it  receives  from  the  water  gas,  which  is  always  dissolved 
therein.    If  two  jars,  filled  with  different  gases,  stand  side  by  side  on  the  shelf,  each 


180 


ANALYSIS  OF  GAS  EVOLVED. 


is  incessantly  disturbing  the  constitution  of  the  other,  nor  does  this  disturbance  cease 
until  the  contents  of  both  jars  are  chemically  the  same.  There  are  some  beautiful  ex- 
periments of  easy  repetition  which  serve  to  show  how  rapidly  gases  and  vapours  can 
thus  percolate  through  fluids.  Take  a  pint  bottle,  and  pass  tlirough  its  cork,  which 
ought  to  fit  it  very  loosely,  a  glass  tube  a  foot  long,  drawn  narrow  at  its  upper  end.  Into 
the  bottle  put  a  few  drops  of  water  of  ammonia.  Dip  the  wide  end  of  the  tube  into  a 
solution  of  soap,  and  introduce  it  into  the  interior  of  the  bottle,  adjusting  it  in  such  a  po- 
sition by  the  cork,  that  when  air  is  blown  in  at  the  narrow  end,  the  soap-bubble  which 
expands  at  the  wide  end  may  occupy  the  middle  of  the  bottle.  Placing  the  lips  on  the 
narrow  end,  blow  a  bubble  an  inch  or  more  in  diameter,  and,  without  loss  of  time,  cau- 
tiously draw  back  again  the  air  from  the  interior  of  the  bubble  into  the  mouth.  A 
strong  ammoniacal  taste  is  at  once  perceived.  Now  it  is  obvious  that  this  ammonia 
must  have  passed  with  very  great  rapidity  through  the  bubble. 

796.  A  still  more  instructive  experiment  may  be  easily  made.  Take  a  three-ounce 
bottle,  with  a  wide  neck,  close  the  mouth  of  it  by  a  film  of  soap-water,  by  passing  the 
moistened  finger  over  it.  Place  it  under  a  jar  of  protoxide  of  nitrogen.  Instantly  the 
horizontality  of  the  film  is  disturbed  ;  it  swells  upward,  and  is  spontaneously  expanded 
by  the  passage  of  the  gas  through  it  into  a  bubble.  The  play  of  colour  which  attends 
this  experiment,  and  the  excessive  thinness  which  the  film  finally  assumes,  render  this 
one  of  the  most  beautiful  experiments  that  chemistry  can  furnish  ;  for  when  the  bubble 
is  almost  invisible  by  reason  of  its  incapacity  to  reflect  light,  and  can  only  be  seen  in 
particular  positions,  it  still  discharges  its  percolating  function. 

797.  This  percolation  of  gases  through  liquids  cannot  be  hindered  by  employing 
oil,  or  such  other  liquids  as  botanical  writers  seem  to  imagine.  Through  common 
lamp-oil,  through  copaiva  balsam,  &c.,  hydrogen  gas  will  escape  with  rapidity,  and  pro- 
toxide of  nitrogen  and  carbonic  acid  still  faster.  The  law  that  regulates  these  phenom- 
ena is  a  very  simple  one  :  the  gas  escapes  through  the  confining  medium  with  a  rapidi- 
ty proportioned  to  its  solubility  therein. 

798.  These  things  being  understood,  it  is  obvious  that  when  carbonic  acid  is  de- 
composed in  the  experiments  we  have  been  detailing,  a  variable  portion  of  that  gas  will 
intermingle  with  the  oxygen  collected.  The  proportions  must  be  variable,  for  it  de- 
pends on  the  amount  of  carbonic  acid  remaining  behind  in  the  water,  on  the  speed  with 
wbich  the  experiment  is  conducted,  and  other  variable  conditions.  As  before  stated, 
therefore,  I  shall  leave  out  of  consideration  this  carbonic  acid,  in  discussing  the  analy- 
sis of  the  collected  gases,  because  it  is  present  by  accident,  and  is  not  essentially  con- 
nected with  the  phenomena,  except  in  one  instance,  where  dark  heat  is  to  be  employed, 
as  will  be  described  presently. 

ANALYSIS  OF  AIR  EVOLVED  FROM  CARBONATED  WATER  BY  THE  SUN. 


Exp. 

Niime  of  Plant. 

Oxygen. 

Nitroijnn. 

1. 

Pinus  taeda  .  .  . 

16  16 

8  34 

2. 

do. 

2716 

13-84 

3. 

do. 

22-33 

21C7 

4. 

Poa  annua  .  .  . 

9000 

10  00 

5. 

do. 

77  90 

22  10 

799.  I  may  remark,  that  this  table  contains  a  few  out  of  a  great  number  of  experi- 


DECOMPOSITION  OF  SALINE  BODIES.  jg]^ 

ments,  all  of  which  might  have  been  quoted  as  examples  of  the  observations  which  I 
wish  to  deduce  from  it.  1st.  They  all  coincide  in  this  respect,  that  the  oxjgen  is  never 
evolved  without  the  simultaneous  appearance  of  nitrogen.  2d.  That  when  certain 
leaves  are  euiplojed,  as  those  of  the  Piims  tceda,  there  seems  to  be  a  very  simple  rela- 
tion between  the  volumes  of  oxygen  and  nitrogen.  In  the  first  and  second  of  those  ex- 
periments, the  volume  of  the  oxygen  is  to  that  of  the  nitrogen  as  two  to  one;  in  the  third, 
as  one  to  one.  In  certain  cases  this  apparent  simplicity  of  proportion  is  departed  from; 
but  from  its  frequent  occurrence  in  many  analyses  I  have  made,  it  seems  to  demand  at- 
tentive consideration.  Moreover,  in  other  plants,  as  in  experiments  4  and  5,  the  amount 
of  oxygen  is  relatively  greater,  and  between  it  and  the  nitrogen  there  does  not  appear 
any  exact  proportion. 

800.  In  order  to  ascertain  whether  decompositions  taking  place  under  absorbent  me- 
dia, as  bichromate  of  potash,  produce  the  same  results  as  indicated  in  the  foregoing  ta- 
ble, I  made  several  analyses  of  gas  collected  under  these  circumstances.  The  presence 
of  the  absorbent  medium  did  not  seem  to  exert  any  influence  whatever,  the  general  re- 
sults coming  out  as  though  it  had  not  been  employed. 

801.  It  has  long  been  a  matter  of  popular  observation  that  the  sunlight  has  the  qual- 
ity of  extinguishing  domestic  fires.  I  do  not  know  whether  there  is  in  reality  any 
ground  for  this  opinion  ;  or  if  so,  whether  the  phenomenon  is  in  any  way  connected 
with  the  relations  of  light  to  carbon  and  oxygen.  Popular  opinion  ascribes  the  effect 
to  the  light,  and  not  the  heat  of  the  ray.  To  determine  whether  radiant  heat,  unaccom- 
panied by  light,  had  the  power  of  producing  the  decomposition  of  carbonic  acid  through 
the  agency  of  leaves,  I  placed  in  the  focus  of  a  large  brass  concave  mirror  a  vessel  con- 
taining some  pine  leaves  in  carbonated  water.  The  mirror  was  set  before  a  wood  fire, 
and  after  a  little  time  the  leaves  began  evolving  bubbles.  The  temperature  of  the  wa- 
ter rose  as  high  as  140°  Fah.,  and  when  sufficient  gas  was  collected,  examination  proved 
that  nearly  the  whole  of  it  was  absorbed  by  lime  or  potash  water.  From  this,  it  is 
evident  that  radiant  heat  merely  liberates  the  carbonic  acid,  and  does  not  decompose 
it.  This  corroborates,  therefore,  the  result  of  the  prismatic  analysis,  that  it  is  the  light, 
and  not  the  heat,  which  brings  about  the  change. 

802.  Decomposition  of  Alkaline  Salts. — The  conditions  under  which  carbonic  acid 
gas  is  decomposed  being  understood,  I  pass  now  to  the  description  of  similar  decompo- 
sitions occurring  in  the  case  of  saline  bodies.  It  has  always  been  a  subject  of  surprise 
to  chemists,  that  the  powerful  affinity  by  which  carbon  and  oxygen  are  held  together 
should  be  so  easily  overcome  at  common  temperatures.  Even  potassium  cannot  de- 
compose carbonic  acid  in  the  cold.  It  might,  therefore,  be  reasonably  expected  that  the 
energetic  forces  which  bring  about  this  change  ought  also  to  effect  other  remarkable 
decompositions.  In  fact,  as  I  shall  now  proceed  to  show,  the  decomposition  of  carbonic 
acid  is  only  one  of  a  very  numerous  series. 

803.  The  alkaline  bicarbonates,  as  is  well  known,  undergo  decomposition  by  a  slight 
elevation  of  temperature.  When  boiled  with  water  they  gradually  give  off  their  second 
atom  of  acid,  and  slowly  pass  into  the  condition  of  neutral  carbonate.  This  easy  de- 
composibility  led  me  to  inquire  whether  green  leaves,  under  the  action  of  the  sunlight, 


182 


DECOMPOSITION  OF  SALINK  BODIES. 


would  effect  the  liberation  and  subsequent  reduction  of  the  acid.  In  the  following  ex-, 
periments  it  is  to  be  observed,  that  the  boiling  is  not  continued  long  enough  to  affect 
to  any  extent  the  constitution  of  the  salt,  and  in  each  case  any  portion  of  free  carbonic 
acid  extricated  during  the  cooling  of  the  liquid  was  removed  by  the  action  of  the  air- 
pump.  The  solution,  when  finally  used,  contained  no  gaseous  matter,  but  only  the  salt 
dissolved  in  water. 

804.  Having  boiled  some  distilled  water  to  expel  all  gaseous  matter,  dissolve  in  it  a 
small  quantity  of  bicarbonate  of  soda.  Introduce  into  a  test  tube  some  leaves  of  grass, 
fill  the  tube  with  the  saline  solution,  which  has  been  once  more  boiled  to  expel  any  air 
it  may  have  obtained  from  the  dissolving  salt,  and  invert  the  tube  in  some  of  the  solu- 
tion in  a  wine-glass,  after  having  carefully  removed  all  adhering  bubbles  of  air  from  the 
leaves  by  a  piece  of  wire,  or  in  any  other  convenient  manner.  This  arrangement,  kept 
in  the  dark,  undergoes  no  change  ;  but,  if  brought  into  the  sunshine,  bubbles  of  gas  are 
rapidly  evolved,  and  in  the  course  of  a  few  hours  the  tube  becomes  half  full.  On  de- 
tonation with  hydrogen  this  gas  proves  to  be  rich  in  oxygen. 

805.  I  made  some  attempts  to  discover  how  much  oxygen  could  in  this  way  be 
evolved  from  known  quantities  of  bicarbonate  of  soda,  supposing  it  probable  that  the 
second  atom  of  carbonic  acid  being  removed  and  decomposed,  the  process  would  cease. 
I  need  not  detail  the  result  of  those  trials ;  they  indicated  that  the  supposition  I  had 
formed  was  not  correct.  The  process  is  not  limited  to  the  removal  and  decomposition 
of  the  second  atom,  but  goes  forward,  the  first  atom  itself  being  in  like  manner  decom- 
posed. From  this  it  would  seem  that  carbonate  of  soda  itself  should  be  decomposed, 
and  experiment  verifies  the  conclusion ;  for,  on  using  that  salt  instead  of  the  bicarbo- 
nate, the  evolution  of  oxygen  goes  on  precisely  in  the  same  way. 

806.  As  in  these  experiments  solid  salt  dissolved  in  water  is  decomposed,  it  is  obvious 
that  the  function  by  which  the  leaves  accomplish  this  is  very  different  from  that  of  res- 
piration.   It  is  not  respiration,  but  a  true  digestion. 

807.  LiEBiG  has  shown  that  ammonia  exists  in  the  ascending  sap.  It  is  probable, 
therefore,  that  it  does  not  undergo  final  change  before  reaching  the  upper  face  (sky- 
face)  of  the  leaf  There,  if  it  be  in  the  form  of  a  carbonate,  it  unquestionably  is  con- 
cerned in  decomposition.  With  the  natural  experiment  before  us,  we  might  expect 
that  the  carbonate  of  ammonia  used  in  place  of  the  soda  salts  of  the  last  experiment 
would  yield  like  them.  Accordingly,  it  will  be  found,  by  using  the  officinal  sesqui- 
carbonate  of  anunonia,  that  leaves  effect  its  decomposition.  In  numerous  experiments 
it  has  yielded  me  gas  frequently  containing  more  than  90  per  cent,  of  oxygen. 

808.  In  every  instance  which  I  have  examined,  the  gas  evolved  from  leaves  is  not 
pure  oxygen,  but.  as  has  been  said,  a  variable  mixture  of  oxygen  and  nitrogen.  This 
result  is  of  uniform  occurrence ;  I  have  observed  it  in  low  latitudes,  where  the  sun  is 
extremely  brilliant,  in  the  case  of  different  plants ;  and  on  referring  to  Dr.  Daubeny's 
paper  ,it  will  appear  that  he  has  uniformly  recognised  the  same  result  in  England. 
The  very  remarkable  qualities  which  certain  nitrogenized  substances  are  known  to 
exhibit,  acting  as  ferments  as  they  are  undergoing  decay,  might  lead  to  the  suspicion 
that  the  decomposition  of  carbonic  acid  by  leaves  is  due  to  the  action  of  some  nitro- 
genized body,  the  eremacausis  of  which  is  promoted  by  the  rays  of  the  sun. 


PRODUCTION  OF  NITROGEN.  183 

809.  There  are  many  facts  which  go  to  prove  that  the  decomposition  of  carbonic 
acid  is  a  secondary  result,  brought  about  by  the  action  of  nitrogenized  ferment  in  a  state 
of  eremacausis,  the  sunlisht  operating  in  the  first  instance  upon  the  ferment  itself 
Plants  can  grow  in  a  certain  manner  in  dark  places,  and  if  the  observations  of  botanists 
have  been  correctly  made,  although  this  kind  of  growth  may  be  abnormal,  it  eventuates 
in  increasing  the  total  weight  of  carbon.  It  signifies  little  that  in  these  instances  lignin 
may  often  be  deficient,  for  other  bodies  of  the  starch  family  make  their  appearance ; 
and  results  of  this  kind  serve  to  show  that,  though  in  all  ordinary  cases  the  union  of  car- 
bon with  the  elements  of  water  is  an  effect  of  light,  there  are  other  cases  in  which,  either 
by  ferment  action,  or  other  powers  residing  in  the  plant,  the  same  result  can  be  attained. 

810.  BoussiNGAULT  statcs  that  grass  leaves  dried  in  air  at  212°  Fah.,  and  burned  with 
oxide  of  copper,  yield  1-3  per  cent,  of  their  dry  weight  of  nitrogen,  which  nitrogen  is, 
of  course,  in  combination.  I  found,  however,  that  there  is,  besides  this,  included  in  the 
tissue  of  the  leaf  a  certain  quantity  of  gas,  which  can  be  removed  by  the  air-pump.  I 
presume  this  air  is  naturally  enclosed  in  the  spiral  vessels.  When  leaves  are  placed  in 
an  inverted  jar  with  boiled  water  in  vacuo,  this  gas  is  liberated ;  at  first,  most  copiously 
from  the  fractured  extremity;  but  as  the  process  of  exhaustion  goes  on,  it  exudes  from 
both  faces  of  the  leaf,  perhaps  by  rending  open  the  frail  tissue  in  which  it  is  imprisoned. 
In  leaves  that  have  stomata  on  one  side  only,  it  does  not  pour  forth  from  those  organs 
in  preference  to  other  parts,  and  from  this  it  may  be  inferred  that  it  does  not  normally 
exist  in  the  intercellular  spaces.  In  a  given  weight  of  leaves  its  amount  is  very  varia- 
ble, ranging,  in  my  experiments,  from  -01  to  -02  cubic  inch  for  ten  grains  of  grass  leaves. 
Its  constitution,  as  determined  by  analysis,  is  also  variable,  but  very  remarkable ;  it  con- 
tains from  88  to  94  per  cent,  of  nitrogen. 

811.  It  being,  therefore,  understood  that  in  the  tissue  of  the  leaf  a  certain  quantity 
of  gas  is  mechanically  included,  which  gas  differs  from  atmospheric  air  in  the  circum- 
stance that  it  contains  a  larger  volume  of  nitrogen,  which  may  be  removed  by  the  air- 
pump,  we  are  in  a  condition  to  understand  whether  it  is  this  nitrogen  which  furnishes 
the  supply  found  in  the  gas  exhaled  by  leaves.  The  following  experiment  proves  that 
it  is  not : 

812.  I  removed  by  continued  boiling  and  exhaustion  all  the  air  dissolved  in  a  solu- 
tion of  bicarbonate  of  soda.  I  also  removed  all  the  nitrogen  from  some  grass  leaves, 
by  placing  them  in  vacuo  immersed  in  water  that  had  been  boiled  and  subsequently 
cooled.  Then  placing  these  leaves  in  the  solution  of  the  bicarlionate  and  in  the  ves- 
sels in  which  the  experiment  was  finally  to  be  conducted,  I  kept  them  in  vacuo  for  an 
hour.  This  was  done  to  get  rid  of  that  film  of  atmospheric  air  which  always  adheres 
to  the  surface  of  glass  vessels,  and  which  might  have  disturbed  the  result  by  furnishing 
nitrogen.  The  leaves  were  now  exposed  in  the  saline  solution  to  the  beams  of  the 
sun,  and  presently  the  evolution  of  gas  commenced.  When  a  sufficient  quantity  was 
collected,  it  was  found  to  consist  of  88  per  cent,  oxygen  and  12  nitrogen. 

813.  Repetitions  of  this  experiment  prove  that,  although  the  nitrogen  mechanically 
enclosed  in  the  leaf  to  a  certain  extent  mingles  with  the  oxygen  evolved,  and,  indeed, 
it  could  not  be  otherwise  on  account  of  the  diffusion  of  gases  into  one  another,  yet  the 


184 


DISAPPEARANCE  OF  OXYGEN. 


true  source  is  to  be  sought  in  some  nitrogenized  compound  present  in  the  leaf,  which 
is  undergoing  decomposition  in  a  regulated  way. 

814.  Keeping  this  l"act  clearlj  before  us,  that  the  source  of  the  nitrogen  found  thus 
in  company  with  the  oxygen,  given  off  under  the  influence  of  light,  is  some  nitrogeni- 
zed body  existing  in  the  leaf,  the  following  experiments  wall  show  the  simple  and  beau- 
tiful law  under  which  this  phenomenon  is  conducted. 

815.  Saussure  has  already  determined  that  when  plants  are  forced  to  grow  in  an 
atmosphere  of  known  volume,  containing  carbonic  acid  gas,  after  the  decomposition 
of  the  gas  is  completed,  the  total  volume  remains  unchanged.  As  my  experiments  were 
made  with  leaves  immersed  in  water,  I  was  desirous  of  proving  whether,  under  these 
forced  circumstances,  the  same  result  would  still  hold  good. 

816.  To  a  certain  quantity  of  water,  from  which  all  air  had  been  expelled,  confined 
in  ajar  over  mercury,  I  passed  20  measures  of  carbonic  acid  gas;  by  a  little  agitation 
the  water  took  up  15-50  measures  of  the  acid.  I  now  introduced  into  the  jar  some 
leaves,  taking  the  greatest  care  that  no  bubbles  of  air  should  pass  along  with  them. 
The  jar  was  then  placed  in  the  sunshine,  and  the  decomposition  completed.  Corrected 
for  variation  of  temperature  and  pressure,  the  resulting  volume  of  the  gas  in  two  experi- 
ments was  20,  or  precisely  the  same  as  that  of  the  carbonic  acid. 

817.  We  may  therefore  infer  that  the  volume  of  mixed  gases  evolved  is  precisely 
equal  to  the  volume  of  carbonic  acid  that  disappears.  This  leads  us  to  some  very  re- 
markable conclusions. 

818.  When  the  leaves  of  plants,  under  the  influence  of  light,  decompose  carbonic 
acid,  they  assimilate  all  the  carbon,  and  a  certain  proportion  of  oxygen  disappears,  at 
the  same  time  they  emit  a  volume  of  nitrogen  equal  to  that  of  the  oxygen  consumed. 

819.  This  disappearance  of  oxygen  and  appearance  of  nitrogen  are  thus  connected 
with  each  other  :  they  are  equivalent  phenomena. 

820.  The  emission  of  nitrogen  is  thus  shown  not  to  be  a  mere  accidental  result, 
but  to  be  profoundly  connected  with  the  whole  physiological  action. 

821.  I  arrive  also  at  this  conclusion  from  experiments  of  another  kind.  If  the  nitro- 
gen that  appears  in  company  with  oxygen  were  obtained  by  diflhsion  from  gas  mechan- 
ically shut  up  in  the  parenchyma  of  the  leaf,  it  is  plain,  in  the  mode  of  operation  which 
I  have  followed,  in  which  leaves  are  immersed  under  water,  and  no  opportunity  given 
them  of  restoring  their  mechanically  included  air,  if  it  were  by  any  means  withdrawn, 
that  the  first  portions  of  mixed  gas  evolved  should  be  richest  in  nitrogen,  and  that  the 
per-centage  amount  should  gradually  become  less  and  less,  as  it  was  removed  from  the 
structure  of  the  leaf:  this  follows  from  the  laws  of  the  diffusion  of  gases.  But  this  is 
far  from  being  the  case.  It  very  commonly  happens  that  more  nitrogen  is  evolved  at 
the  close  of  the  process  than  at  its  beginning.  Thus,  in  one  of  the  experiments  I  made, 
in  which  it  was  found  that  there  was  22-2  per  cent,  of  nitrogen  in  the  total  resulting 
volume,  the  quantities  that  had  been  evolved  in  three  successive  periods  of  examination, 
from  the  beginning  to  the  termination  of  the  experiment,  were, 

1st  period,  21-8  per  cent,  of  nitrogen. 
2d      "  18-8 
3d      "  26-0 


I 


TITHONOTYPES  IN  COPPER.  185 

During  the  progress  of  this  decomposition,  therefore,  more  nitrogen,  relatively,  was 
evolved  towards  the  close  of  the  experiment  than  at  its  beginning. 

822.  From  this  result,  therefore,  I  again  infer  that  the  nitrogen  emitted  by  leaves  is 
derived  from  the  decomposition  of  some  azotized  body,  and  not  from  air  mechanically 
included  in  their  pores. 

823.  The  following  are  the  experimental  results  which  I  have  obtained  : 

iGt.  That  the  nitrogen  comes  from  the  tissue  of  the  leaf  itself ;  because  more  than 
three  times  as  much  is  evolved  from  bicarbonate  of  soda  as  is  imprisoned  in  the  struc- 
ture of  the  leaf,  removable  by  the  air-pump. 

2d.  In  twelve  hours,  from  bicarbonate  of  soda,  leaves  will  evolve  more  than  five 
times  their  own  volume  of  gaseous  matter. 

3d.  The  quantity  of  nitrogen  in  the  composition  of  leaves  is  sufficient  for  furnishing 
all  the  nitrogen  obtained  in  the  gas  evolved.  From  Boussingault's  analyses  it  ap- 
pears that  they  contain  nearly  ten  times  the  required  amount. 

4th.  The  decomposition  of  some  nitrogenized  constituent  of  the  leaf  is  essential  to 
the  appearance  of  the  nitrogen  :  there  is  no  other  available  source. 

824.  At  this  stage  of  the  inquiry  a  remarkable  analogy  appears  between  the  func- 
tion of  digestion  in  animals  and  the  same  function  in  plants.  Liebig  has  shown  how, 
from  the  transformation  of  the  stomach  itself,  food  becomes  acted  upon  and  is  turned  into 
chyme :  an  obscure  species  of  fermentation,  brought  about  by  the  action  of  nitrogen- 
ized bodies.  So,  in  like  manner,  in  plants,  the  decay  of  a  nitrogenized  body  is  inti- 
mately connected  with  the  assimilation  of  carbon;  for,  as  I  have  stated,  the  process 
here  under  discussion  is  a  true  digestion  and  not  a  respiratory  process.  And  as  there 
are  facts  which  seem  to  show  that  the  primary  action  of  the  light  is  not  upon  the  car- 
bonic acid,  but  upon  the  nitrogenized  ferment,  the  decomposition  of  the  gas  ensuing 
as  a  secondary  result,  is  it  not  probable  that  chlorophyl  is  the  body  which  in  veg- 
etables answers  to  the  chyle  of  animals  I  The  oxygen,  which  disappears  during  the 
decomposition  of  carbonic  acid,  disappears  to  bring  about  the  eremacausis  of  the  nitro- 
genized body.  And  have  not  the  gum,  the  starch,  the  lignin,  and  other  carbonaceous 
constituents  of  plants,  all  originally  existed  in  and  passed  through  the  green  stage  ?  It 
is  the  quality  of  radiant  matter  to  determine  the  position  of  atoms  and  the  grouping  of 
molecules  ;  and  for  this  the  sun,  the  great  organizer,  the  great  life-giver,  from  age  to 
age  furnishes  his  unfading  beams.  That  analogies  like  this  between  the  organic  func- 
tions of  plants  and  animals  in  reality  exist,  we  might  reasonably  suppose ;  they  are 
agreeable  to  the  general  plan  of  nature. 


Note  on  the  Tithonotype. 

825.  In  chapter  xiv.  I  described  a  process  for  obtaining  tithonotypes,  or  copies  of  the 
surface  of  Daguerreotypes,  by  means  of  gelatine. 

826.  A  very  important  improvement  on  that  process — an  improvement  which,  indeed, 
has  brought  it  almost  at  once  to  perfection — has  been  effected  ;  this  is.  To  copy  the  sur- 

Aa 


186 


DETITHONIZING  POWER  OF  GASES. 


face  in  copper  hy  the  electrotype  after  it  has  been  previously  fixed  by  the  agency  of  a  film 
of  gold. 

827.  Those  who  are  conversant  with  these  matters  will  see  at  once  that  this  is  a  very 
different  thing  from  the  abortive  attempts  which  were  made  early  in  the  history  of  the 
Daguerreotype.  Many  artists  endeavoured  to  transfer  its  surface  by  precipitating  cop- 
per upon  it ;  among  others,  I  made  trials  of  the  kind.  The  results  of  those  abortive 
attempts  were  mere  shadowy  representations,  which  could  be  seen  in  certain  lights, 
and  which  were  very  unsatisfactory  in  their  effect.* 

828.  The  beautiful  tithonotypes  that  are  now  so  connnon  in  this  city  are  made  in 
the  following  way  :  The  Daguerreotype  is  carefully  gilt  by  M.  Fizeau's  process,  taking 
care  that  the  film  of  gold  is  neither  too  thick  nor  too  thin.  The  proper  thickness  is 
readily  attained  after  a  little  practice.  The  j)late  is  then  kept  a  day  or  two,  so  that  it 
may  become  enfilmed  with  air.  The  back  and  edges  being  varnished,  copper  is  to  be 
deposited  upon  it  in  the  usual  way,  the  process  occupying  from  twelve  to  twenty  hours. 
If  the  plate  has  been  properly  gilt,  and  the  process  conducted  successfully,  the  tithono- 
type  readily  splits  off  from  the  Daguerreotype. 

829.  The  reader  will  understand  that,  when  the  process  succeeds,  the  Daguerreotype 
will  be  uninjured,  and  the  tithonotype  a  perfect  copy  of  it.  If  any  portions  are  blue, 
or  white,  or  flesh-coloured,  they  will  be  seen  in  the  same  colours  in  the  tithonotype ; 
the  intensity  of  light  and  shadow  is  also  given  with  accuracy,  and,  indeed,  the  copy  is 
a  perfect  copy,  in  all  respects,  of  the  original.  A  great  advantage  is  also  obtained  in 
the  reversal  that  takes  place.  The  right  side  of  the  tithonotype  corresponds  to  the  right 
side  of  the  original  object,  and  the  left  to  the  left.    In  the  Daguerreotype  it  is  not  so. 

830.  Copper  tithonotypes  were  first  made  in  this  city  by  Mr.  Endicott,  a  litho- 
graphic artist  of  distinction. 

831.  There  is  no  great  difficulty  in  obtaining  from  these  tithonotypes  duplicate  copies. 
An  expert  artist  can  multiply  them  from  one  another. 

832.  The  problem  of  multiplying  the  beautiful  productions  of  M.  Daguerre  is  there- 
fore solved. 

833.  I  will  take  this  opportunity  of  making  a  remark  which  I  intended  to  have  in- 
serted in  my  paper  "  On  the  rapid  Dctithonizing  power  of  certain  Gases  and  Vapours," 
inserted  in  the  March  number  of  the  Philosophical  Magazine  (S.  3,  vol.  xxii.).  Amateurs 
in  the  Daguerreotype  process  are  often  annoyed  by  the  want  of  success  which  fre- 
quently attends  them.  They  ascribe  to  the  atmosphere,  or  to  the  light,  or  to  other 
causes,  their  inability  to  obtain  impressions.  Most  of  these  mischances  are  due  to  the 
accidental  presence  of  the  vapour  of  iodine,  or  other  electro-negative  bodies,  in  the 
chamber  or  about  the  apparatus.  It  is  incredible  what  a  brief  exposure  to  these  va- 
pours will  entirely  destroy  a  picture  before  it  is  mercurialized.  If  the  iodine  box  or  the 
bromine  bottle  is  kept  in  the  same  room  with  the  mercury  apparatus,  that  circumstance 
in  itself  is  often  sufficient  to  ensure  a  uniform  want  of  success.  If  the  little  frame  whi(;h 
fits  into  the  back  of  the  camera,  and  which  holds  the  silver  plate,  be  used  in  the  iodi- 


♦  Professor  Grove's  voltaic  process  for  etching  Daguerreotypes  has,  however,  produced  better  results  than  those  here 
alluded  to  by  Dr.  Draper.    See  Phil.  Mag.,  S.  3,  vol.  xx  ,  p.  18. — Edit.  Phil.  Mao. 


THE  INSTRUMENT  CONSISTS  OF  A  MIXTURE  OF  CHLORINE  AND  HYDROGEN.     2  87 


zing  process,  as  is  often  the  case,  the  small  quantity  of  vapour  it  absorbs  will  destroy 
every  picture,  or,  at  all  events,  increase  the  time  required  in  the  camera  enormously. 
The  reason  of  this  is  easily  understood.  Suppose  a  plate  in  such  a  frame  be  placed  in 
the  camera,  or,  what  comes  to  the  same  thing,  suppose  a  particle  of  iodine  has  fallen 
into  the  camera,  or  that  the  wood  has  in  any  way  absorbed  an  electro-negative  vapour; 
as  fast  as  the  light  makes  its  impression  on  the  sensitive  surface  the  vapour  detithonizes 
it,  and  unless  the  light  is  quite  intense  or  the  exposure  much  prolonged,  a  very  feeble 
proof,  or  no  proof  at  all,  will  be  obtained.  In  the  same  way,  the  difficulties  are  greatly 
increased  in  the  process  of  mercurialization ;  for  the  temperature  resorted  to  being  high, 
if  there  is  the  least  particle  of  iodine  about  the  box,  the  picture  will  be  inevitably  and 
instantly  detithonized  and  ruined. 

834.  We  ought,  therefore,  never  to  allow  iodine,  or  bromine,  or  chlorine,  to  have  ac- 
cess to  the  apartment  or  the  apparatus  in  which  Daguerreotype  operations  are  being 
conducted. 


CHAPTER  XVL 

DESCRIPTION  OF  THE   TITHONOMETER,  AN  INSTRUMENT  FOR  MEASURING  THE  CHEMICAL 

FORCE   OF   THE   INDIGO-TITHONIC  RAYS. 

{From  the  London,  Edinburgh,  and  Dublin  Philosophical  Magazine  for  December,  1843.) 

Contents  :  The  Instrument  consists  of  a  Mixture  of  Chlorine  and  Hydrogen. — It  is 
acted  upon  by  Lamp  Light,  an  Electric  Spark  at  aDistance,  ^c. — Chlorine  and  Hydro- 
gen unite  in  Proportion  to  the  Amount  of  Light. — Mode  of  measuring  out  known 
Quantities  of  Rays. —  The  Maxi7num  of  Action  is  in  the  Indigo  Space. — Construction 
of  the  Instrument. —  Theoretical  Conditions  of  Equilihrium. — Preliminary  Adjust- 
ment. — Method  of  continuous  Ohservation. — Method  of  interrupted  Ohsertation. — 
Remarkable  Contraction  and  Expansion. 

835.  I  HAVE  invented  an  instrument  for  measuring  the  chemical  force  of  the  tithonic 
rays  which  are  found  at  a  maximum  in  the  indigo  space,  and  which  from  that  point 
gradually  fade  away  to  each  end  of  the  spectrum.  The  sensitiveness,  speed  of  action, 
and  exactitude  of  this  instrument,  will  bring  it  to  rank,  as  a  means  of  physical  research, 
with  the  thermo-multiplier  of  M.  Melloni. 

836.  The  means  which  have  hitherto  been  found  available  in  optics  for  measuring 
intensities  of  light,  by  a  relative  illumination  of  spaces  or  contrast  of  shadows,  are  ad- 
mitted to  be  inexact.  The  great  desideratum  in  that  science  is  a  photometer  which 
can  mark  down  effects  by  movements  over  a  graduated  scale.  With  those  optical 
contrivances  may  be  classed  the  methods  hitherto  adopted  for  determining  the  force  of 
the  tithonic  rays  by  stains  on  Daguerreotype  plates,  or  the  darkening  of  sensitive  pa- 
pers. As  deductions  drawn  in  this  way  depend  on  the  opinion  of  the  observer,  they 
can  never  be  perfectly  satisfactory,  nor  bear  any  comparison  with  thennometric  results. 


188 


IT  IS  ACTED  ON  BY  LAMPLIGHT. 


837.  Impressed  with  the  importance  of  possessing,  for  the  study  of  the  properties  of 
the  tithonic  rajs,  some  means  of  accurate  measurement,  I  have  resorted  in  vain  to  many 
contrivances ;  and,  after  much  labour,  have  obtained  at  last  the  instrument  which  it  is 
the  object  of  this  paper  to  describe. 

838.  The  tithononieter  consists,  essentially,  of  a  mixture  of  equal  measures  of  chlo- 
rine and  hydrogen  gases,  evolved  from  and  confined  by  a  fluid  which  absorbs  neither. 
This  mixture  is  kept  in  a  graduated  tube,  so  arranged  that  the  gaseous  surface  exposed 
to  the  rays  never  varies  in  extent,  notwithstanding  the  contraction  which  may  be  going 
on  in  its  volume,  and  the  muriatic  acid  resulting  from  its  union  is  removed  by  rapid 
absorption. 

839.  The  theoretical  conditions  of  the  instrument  are,  therefore,  sufficiently  simple; 
but,  when  we  come  to  put  them  into  practice,  obstacles  which  appear  at  first  sight  insur- 
mountable are  met  with.  The  means  of  obtaining  chlorine  are  all  troublesome  ;  no 
liquid  is  known  which  will  perfectly  confine  it ;  it  is  a  matter  of  great  difficulty  to  mix 
it  in  the  true  proportion  with  hydrogen,  and  have  no  excess  of  either.  Nor  is  it  at  all 
an  easy  afiair  to  obtain  pure  hydrogen  speedily,  and  both  these  gases  difiuse  with  ra- 
pidity through  water  into  air. 

840.  Without  dwelling  farther  on  the  long  catalogue  of  difficulties  which  is  thus  to 
be  encountered,  I  shall  first  give  an  account  of  the  capabilities  of  the  instrument  in  the 
form  now  described,  which  will  show  to  what  an  extent  all  those  difficulties  are  al- 
ready overcome.  In  a  course  of  experiments  on  the  union  of  chlorine  and  hydro- 
gen, some  of  which  were  read  at  the  last  meeting  of  the  British  Association,  I  found 
that  the  sensitiveness  of  that  mixture  had  been  greatly  underrated.  The  statement 
made  in  the  books  of  chemistry,  that  artificial  light  will  not  affect  it,  is  ^\  holly  erro- 
neous. The  feeblest  gleams  of  a  taper  produce  a  change.  No  farther  proof  of  this 
is  required  than  the  tables  given  in  this  chapter,  in  which  the  radiant  source  was  an 
oil-lamp.  For  speed  of  action,  no  tithonographic  compound  can  approach  it ;  a  light 
which  perhaps  does  not  endure  the  millionth  part  of  a  second,  affects  it  energetically, 
as  will  be  hereafter  shown. 

841.  Proofs  of  the  Sensitiveness  of  the  Tithonometer. — The  following  illustrations 
will  show  that  the  tithonometer  is  promptly  affected  by  rays  of  the  feeblest  intensity, 
and  of  the  briefest  duration. 

842.  When,  on  the  sentient  tube  of  the  tithonometer,  the  image  of  a  lamp  flame 
formed  by  a  convex  lens  is  caused  to  fall,  the  liquid  instantly  begins  to  move  over  the 
scale,  and  continues  its  motion  as  long  as  the  exposure  is  continued.  It  does  not 
answer  to  expose  the  tube  to  the  direct  emanations  of  the  lamp  without  first  absorbing 
the  radiant  heat,  or  the  calorific  effect  will  mask  the  true  result.  By  the  interposition 
of  a  lens  this  heat  is  al)sorbed,  and  the  tithonic  rays  alone  act. 

843.  If  a  tithonometer  is  exposed  to  daylight  coming  through  a  window,  and  the  hand, 
or  a  shade  of  any  kind,  is  passed  in  front  of  it,  its  movement  is  in  cm  instant  arrested  ; 
nor  can  the  shade  be  passed  so  rapidly  that  the  instrument  will  fail  to  give  the 
proper  indication. 

844.  The  experimenter  may  farther  assure  himself  of  the  extreme  sensitiveness  of 


AND  BY  AN  ELECTRIC  SPARK  AT  A  DISTANCE.  Jgg 

this  mixture  by  placing  the  instrument  before  a  window,  and  endeavouring  to  remove 
and  replace  its  screen  so  quickly  that  it  shall  fail  to  give  any  indication ;  he  will  find 
that  it  cannot  be  done. 

845.  Charge  a  Leyden  vial,  and  place  the  tithonometer  at  a  little  distance  from  it, 
keeping  the  eye  steadily  lixed  on  the  scale ;  discharge  the  jar,  and  the  rays  from  the 
spark  will  be  seen  to  exert  a  very  powerful  effect,  the  movement  taking  place  and  ceas- 
ing in  an  instant. 

846.  This  remarkable  experiment  not  only  serves  to  prove  the  sensitiveness  of  the 
tithonometer.  but  also  brings  before  us  new  views  of  the  powers  of  that  extraordinary 
agent,  electricity.  That  energetic  chemical  effects  can  thus  be  produced  at  a  distance 
by  an  electric  spark  in  its  momentary  passage,  effects  which  are  of  a  totally  different 
kind  from  the  common  manifestations  of  electricity,  is  thus  proved  ;  these  phenomena 
being  distinct  from  those  of  induction  or  molecular  movements  taking  place  in  the  line 
of  discharge,  they  are  of  a  radiant  character,  and  due  to  the  emission  of  tithonicity  ; 
and  we  are  led  at  once  to  infer  that  the  well-known  changes  brought  about  by  passing 
an  electric  spark  through  gaseous  mixtures,  as  when  oxygen  and  hydrogen  are  com- 
bined into  water,  or  chlorine  and  hydrogen  into  muriatic  acid,  arise  from  a  very  differ- 
ent cause  than  those  condensations  and  percussions  by  which  they  are  often  explained, 
a  cause  far  more  purely  chemical  in  its  kind.  If  chlorine  and  hydrogen  can  be  made 
to  unite  silently  by  an  electric  spark  passing  outside  the  vessel  which  contains  them,  at 
a  distance  of  several  inches,  there  is  no  difficulty  in  understanding  why  a  similar  effect 
should  take  place  with  a  violent  explosion  when  the  discharge  is  made  through  their 
midst,  nor  how  a  great  many  mixtures  may  be  made  to  unite  under  the  same  treat- 
ment. A  flash  of  lightning  cannot  take  place,  nor  an  electric  spark  be  discharged, 
without  chemical  changes  being  brought  about  by  the  radiant  matter  emitted. 

847.  Proofs  of  the  Exactness  of  the  Indications  of  the  Tithonometer. — The  foregoing 
examples  may  serve  to  illustrate  the  extreme  sensitiveness  of  the  tithonometer;  I  shall 
next  furnish  proofs  that  its  indications  are  exactly  proportional  to  the  quantities  of  light 
incident  on  it. 

848.  As  it  is  necessary,  owing  to  the  variable  force  of  daylight,  to  resort  to  artifi- 
cial means  of  illumination,  it  will  be  found  advantageous  to  employ  the  following  method 
of  obtaining  a  flame  of  suitable  intensity  : 

849.  Let  A  B  {^fig.  104)  be  an  Argand  oil  lamp,  of  which  the  wick  is  C.  Over  the 
wick,  at  a  distance  of  half  an  inch,  or  thereabout,  place  a  plate  of  thin  sheet  copper, 
three  inches  in  diameter,  perforated  in  its  centre  with  a  circular  hole  of  the  same  di- 
ameter as  the  wick,  and  concentric  therewith.  This  piece  of  copper  is  represented  at 
d  d;  it  should  have  some  contrivance  for  raising  or  depressing  it  through  a  small  space, 
the  proper  height  being  determined  by  trial.  On  this  plate  the  glass  cylinder,  e,  an  inch 
and  three  quarters  in  diameter,  and  eight  or  ten  inches  long,  rests. 

850.  When  the  lamp  is  lighted,  provided  the  distance  between  the  plate,  d  d,  and  the 
top  of  the  wick  is  properly  adjusted,  on  putting  on  the  glass  cylinder  the  flame  instantly 
assumes  an  intense  whiteness;  by  raising  the  wick  it  may  be  elongated  to  six  inches  or 
more,  and  becomes  exceedingly  brilliant.    Lamps  constructed  on  these  principles  may 


190 


CHLORINE  AND  HYDROGEN  UNITE  IN  PROPORTION  TO  THE 


be  purchased  in  the  shops.  I  have,  however,  contented  myself  with  using  a  common 
Argand  studj-lamp,  supporting  the  perforated  plate,  d  d,  at 'd  proper  altitude  by  a  retort 
stand.  It  will  be  easily  understood  that  the  great  increase  of  light  arises  from  the  cir- 
cumstance that  the  flame  is  drawn  violently  through  the  aperture  in  the  plate  by  the 
current  established  in  the  cylinder. 

851.  As  nmch  radiant  heat  is  emitted  by  this  flame,  in  order  to  diminish  its  action, 
and  also  to  increase  the  lithonic  effect,  1  adopt  the  following  arrangement:  Let  A  B 
{^/ig.  104)  be  the  lamp:  the  rays  emitted  by  it  are  received  on  a  convex  lens,  D,  four 
inches  and  three  quarters  in  diameter,  that  which  I  use  being  the  large  lens  of  a  lucer- 
nal  microscope.  This,  placed  at  a  distance  of  twenty-one  inches  from  the  lamp,  gives 
an  image  of  the  flame  at  a  distance  of  thirteen  inches,  which  is  received  on  the  sen- 
tient tube  of  the  tithonometer,  F ;  between  the  tithonometer  and  the  lens  there  is  a 
screen,  E. 

852.  Things  being  thus  arranged,  and  the  lamp  lighted  so  as  to  give  a  flame  about 
three  inches  and  a  half  long,  the  experiments  may  be  proceeded  with.  It  is  convenient 
always  to  work  with  the  flame  at  a  constant  height,  which  may  be  determined  by  a 
mark  on  the  glass  cylinder.  At  a  given  instant,  by  a  seconds  watch,  the  screen  E  is 
removed,  and  immediately  the  tithonometer  begins  to  descend.  When  the  first  minute 
is  elapsed,  the  position  on  the  scale  is  read  off  and  registered :  at  the  close  of  the  sec- 
ond minute  the  same  is  done,  and  so  on  with  the  third.  &c.  And  now,  if  those  num- 
bers be  compared,  casting  aside  the  first,  they  will  be  found  equal  to  one  another,  as 
the  following  table  of  experiments,  made  at  different  times  and  with  different  instru- 
ments, shows : 

TABLE  I. 

Showing  that,  when  the  radiant  source  is  constant,  the  amount  of  movement  in  the  tithonometer  is  directly  proportional  to  the  times 

of  exposure. 


Time. 

Experiments. 

1. 

2. 

3. 

4. 

5. 

30" 

700 

7  00 

10  25 

5  25 

60 

8  00 

8-75 

11-50 

11.75 

6  50 

90 

750 

800 

11  50 

6  25 

120 

7.75 

7-75 

11  50 

1300 

600 

150 

7-75 

7-25 

6  00 

180 

12  00 

6  00 

210 

6  00 

Mean 

7  60 

7-55 

1119 

12  25 

6  00 

From  this  it  will  be  perceived  that,  taking  the  first  experiment  as  an  example,  if 
at  the  end  of  30  seconds  the  tithonometer  has  moved  7-00,  at  the  end  of  60"  it  has 
moved  8-00  more ;  at  the  end  of  90",  7-50  more ;  at  the  end  of  120",  7*75  more :  the 
numbers  set  down  in  the  vertical  column  representing  the  amount  of  motion  for  each 
thirty  seconds.  And  when  it  is  recollected  that  the  readings  are  all  made  with  the 
instrument  in  motion,  the  differences  between  the  numbers  do  not  greatly  exceed  the 
possible  errors  of  observation.  It  may  be  remarked  that  the  third  and  fourth  experi- 
ments were  made  with  a  different  lamp. 

853.  Though  a  certain  amount  of  radiant  heat  from  a  source  so  highly  incandescent 
as  that  here  used  will  pass  the  lens,  its  effects  can  never  be  mistaken  for  those  of  the 
tidiojilc  rays.    This  is  easily  understood  when  we  remember  that  the  effect  of  such 


I 


AMOUNT  OF  LIGHT. 


191 


transmitted  heat  would  be  to  expand  the  gaseous  mixture,  but  the  tithonic  effect  is  to 
contract  it. 

854.  Next  I  shall  proceed  to  show  that  the  indications  of  the  tilhonometer  are  strictly 
proportional  to  the  quantity  of  rays  that  have  impinged  upon  it;  a  double  quantity  pro- 
ducing a  doul)le  effect,  a  triple  quantity  a  threefold  effect,  &c. 

855.  A  slight  modification  in  the  arrangement  {Jig.  104)  enables  us  to  prove  tbis  in 
a  satisfactory  way.  The  lens,  D,  being  mounted  in  a  square  wooden  frame,  can  easily 
be  converted  into  an  instrun)ent  for  delivering  at  its  focal  point,  wliere  the  sentient 
tube  is  placed,  measured  quantities  of  the  tithonic  rays,  and  thus  becomes  an  invaluable 
auxiliary  in  those  researches  which  require  known  and  predetermined  quantities  of  titbon- 
icity  to  be  measured  out.  The  principle  of  the  modification  is  easily  apprebended. 
If  half  the  surface  of  the  lens  be  screened  by  an  opaque  body,  as  a  piece  of  blackened 
cardboard,  of  course  only  half  the  quantity  of  rays  will  pass  which  would  have  passed 
had  the  screen  not  been  interposed.  If  one  fourth  of  the  lens  be  left  uncovered,  only 
one  fourth  of  the  quantity  will  pass ;  but  in  all  these  instances  the  focal  image  remains 
the  same  as  before.  By  adjusting,  therefore,  upon  the  wooden  frame  of  the  lens  two 
screens,  the  edges  of  which  pass  through  its  centre,  and  are  capable  of  rotation  upon 
that  centre,  we  shall  cut  off  all  light  when  the  screens  are  applied  edge  to  edge;  we 
shall  have  90°  when  they  are  rotated  so  as  to  be  at  right  angles,  and  180°  when  they 
are  superposed  with  their  edges  parallel.  Thus,  by  setting  them  in  different  angular 
positions,  we  can  gain  all  quantities  from  0°  up  to  180°,  and  by  removing  them  entirely 
away,  reach  360°. 

856.  It  will  be  understood  that  the  effect  of  the  instrument  is  to  give  an  image  of  a 
visible  object,  of  which  the  intensitv  can  be  made  to  vary  at  pleasure  in  a  known  pro- 
portion. 

857.  In  order,  therefore,  to  prove  that  the  indications  of  the  tithonometer  are  pro- 
portional to  the  quantity  of  impinging  rays,  place  this  measuring  lens  in  the  position  U, 
setting  its  screens  at  an  angle  of  90°.  Remove  the  screen  E,  and  determine  the  effect 
on  the  tithonometer  for  one  minute.  At  the  close  of  the  minute,  and  without  loss  of 
time,  turn  one  of  the  screens  so  as  to  give  an  angle  of  180°,  and  now  the  effect  will  be 
found  double  what  it  was  before,  as  in  the  following  table : 

TABLE  ri. 

Showing  that  the  Indications  of  the  Tithonometer  are  proportional  to  the  quantity  of  Incident  Rays. 


Quantities. 

Experiment  1. 

1           Experiment  2. 

Ol;servet]. 

Calculat  '1. 

Observed. 

Calculated. 

90° 

218 

2  22 

2-69 

2-75 

180 

4-27 

4  45 

5-75 

5  .50 

270 

6  70 

6  67 

8  25 

8  25 

360 

8  90 

890 

11  00 

11  00 

858.  I  have  stated  in  the  commencement  of  this  paper,  that  the  action  upon  the 
tithonometer  is  limited  to  a  ray  which  corresponds  in  refrangibility  to  the  indigo,  or, 
rather,  that  in  the  indigo  space  its  maximum  action  is  found.  The  following  table 
serves  at  once  to  prove  this  fact,  and  also  to  illustrate  the  chemical  force  of  the  different 
regions  of  the  spectrum : 


192 


CONSTRUCTION  OF  THE  INSTRUMENT. 


TABLE  III. 

Showing  that  ihe  Maximum  for  the  Tithonometcr  is  in  the  Indigo  Space  of  the  Spectrum. 


Space. 

Ray. 

Force. 

Space. 

Ray. 

Force. 

0 

Extreme  red 

•33 

8 

Blue-indigo  .  . 

204  00 

1 

Red  .    .  . 

■50 

9 

Indigo     .    .  . 

240  00 

2 

Orange  .  . 

•75 

10 

"Violet . 

121  00 

3 

Yellow  .  . 

2-75 

11 

Violet.    .    .  . 

72  00 

4 

Green    .  . 

10  00 

13 

Violet.    .    .  . 

48  00 

5 

Green-blue 

54  00 

13 

Violet.    .    .  . 

24  00 

6 

Blue .    .  . 

10800 

14 

Extra-spectral  . 

1200 

7 

Blue .    .  . 

144  00 

In  this  table  the  spaces  are  equal ;  the  centre  of  the  red,  as  insulated  by  cobalt  blue 
glass,  is  marked  as  unity  ;  the  centre  of  the  yellow,  insulated  by  the  same,  being  marked 
3 ;  the  intervening  region  being  divided  into  two  equal  spaces,  and  divisions  of  the 
same  value  carried  on  to  each  end  of  the  spectrum. 

859.  As  instruments  will  no  doubt  be  hereafter  invented  for  measuring  the  phenom- 
ena of  different  classes  of  rays,  it  may  prove  convenient  to  designate  the  precise  ray  to 
which  they  apply.  Perhaps  the  most  simple  mode  is  to  affix  the  name  of  the  ray  itself. 
Under  that  nomenclature,  the  instrument  described  in  this  paper  would  take  the  name 
of  indigo-tithonometer. 

860.  There  is  no  difficulty  in  adapting  this  instrument  to  the  determination  of  ques- 
tions relating  to  absorption,  reflexion,  and  transmission.  Thus,  I  found  that  a  piece  ot 
colourless  French  plate-glass  transmitted  866  rays  out  of  1000. 

861.  Desa-iption  of  the  Instrument.  First,  of  the  Glass  Part. — The  tithonometcr 
consists  of  a  glass  tube  bent  into  the  form  of  a  siphon,  in  which  chlorine  and  hydro- 
gen can  be  evolved  from  muriatic  acid,  containing  chlorine  in  solution,  by  the  agency 
of  a  voltaic  current.  It  is  represented  hy  Jig.  105.  where  «  c  is  a  clear  and  thin  tube, 
four  tenths  of  an  inch  external  diameter,  closed  at  the  end,  a.  At  d,  a  circular  piece 
of  metal  an  inch  in  diameter,  which  may  be  called  the  stage,  is  fastened  on  the  tube, 
the  distance  from  d  to  a  being  2'9  inches.  At  the  point  x,  which  is  two  inches  and  a 
quarter  from  d,  two  platina  wires,  x  and  y,  are  fused  into  the  glass,  and,  entering  into  the 
interior  of  the  tube,  are  destined  to  furnish  the  supply  of  chlorine  and  hydrogen  ;  from 
the  stage,  d,  to  the  point  b,  the  inner  bend  of  the  tube  is  2-6  inches,  and  from  that 
point  to  the  top  of  the  siphon,  c,  the  distance  is  three  inches  and  a  half  Through 
the  glass  at  z,  three  quarters  of  an  inch  from  c,  a  third  platina  wire  is  passed  ;  this  wire 
terminates  in  the  little  mercury  cup,  r,  and  x  and  y  in  the  cups  2>  and  q  respectively. 

862.  Things  being  thus  arranged,  the  instrument  is  filled  with  its  fluid,  prepared  as 
will  presently  be  described ;  and  as  the  legs,  a  h,h  c,  are  not  parallel  to  each  other,  but 
include  an  angle  of  a  few  degrees,  in  the  same  way  that  Ure's  eudiometer  is  arranged, 
there  is  no  difficulty  in  transferring  the  liquid  to  the  sealed  leg.  Enough  is  admitted 
to  fill  the  sealed  leg  and  the  open  one  partially,  leaving  an  empty  space  to  the  top  of 
the  tube,  at  c,  of  two  and  three  quarter  inches. 

863.  A  stout  tube,  six  inches  long  and  one  tenth  of  an  inch  interior  diameter,  e/  is 
now  fused  on  at  c.  Its  lower  end  opens  into  the  main  siphon  tube  ;  its  upper  end  is 
turned  over  at^^  and  is  narrowed  to  a  fine  termination,  so  as  barely  to  admit  a  pin, 
but  is  not  closed.  This  serves  to  keep  out  dust,  and  in  case  of  a  little  acid  passing  out, 
it  does  not  flow  over  the  scale  and  efface  the  divisions.    At  the  back  of  this  tube  a 


CONSTRUCTION  OF  THE  INSTRUMENT.  293 

scale  is  placed,  divided  into  tenths  of  an  inch,  being  numbered  from  above  downward. 
Fifty  of  these  divisions  are  as  many  as  will  be  required.  Fig.  106  shows  the  termi- 
nation of  the  narrow  tube  bent  over  the  scale. 

864.  From  a  point  one  fourth  of  an  inch  above  the  stage,  d,  downward  beyond  the 
bend,  and  to  within  half  an  inch  of  the  wire,  z,  the  whole  tube  is  carefully  painted  with 
India  ink,  so  as  to  allow  no  light  to  pass;  but  all  the  space  froui  a  fourth  of  an  inch 
above  the  stage,  d,  to  the  top  of  the  tube,  a,  is  kept  as  clear  and  transparent  as  possible. 
This  portion  constitutes  the  sentient  part  of  the  instrument.  A  Hght  metallic  or  paste- 
board cap,  A  D  {^fig.  107),  closed  at  the  top  and  open  at  the  bottom,  three  inches  long 
and  six  tenths  of  an  inch  in  diameter,  blackened  on  its  interior,  may  be  dropped  over 
this  sentient  tube ;  it  being  the  office  of  the  stage,  d,  to  receive  the  lower  end  of  the 
cap  when  it  is  dropped  on  the  tube  so  as  to  shut  out  the  light. 

865.  The  foot  of  the  instrument,  k  I,  is  of  brass ;  it  screws  into  the  hemispherical 
block,  m,  which  may  be  made  of  hard  wood  or  ivory ;  in  this  three  holes,  7?,  q,  r,  are 
made  to  serve  as  mercury  cups ;  they  should  be  deep  and  of  small  diameter,  that  the 
metal  may  not  flow  out  when  it  is  inclined  for  the  purpose  of  transferring.  A  brass  cylin- 
drical cover,  L  M,  L  M,  may  be  put  over  the  whole  ;  when  it  is  desirable  to  preserve 
it  in  total  darkness,  it  should  be  blackened  within. 

866.  Secondly,  of  the  Fluid  Part. — The  fluid  from  which  the  mixture  of  chlorine 
and  hydrogen  is  evolved,  and  by  which  it  is  confined,  is  yellow  comtnercial  muriatic 
acid,  holding  such  a  quantity  of  chlorine  iu  solution  that  it  exerts  no  action  on  the  mixed 
gases  as  they  are  produced.  From  the  mode  of  its  preparation,  it  always  contains  a 
certain  quantity  of  chloride  of  platina,  which  gives  it  a  deep  golden  colour,  a  condition 
of  considerable  incidental  importance. 

867.  When  muriatic  acid  is  decomposed  by  voltaic  electricity,  its  chlorine  is  not 
evolved,  but  is  taken  up  in  very  large  quantity  and  held  in  solution  ;  perhaps  a  bichlo- 
ride of  hydrogen  results.  If  through  such  a  solution  hydrogen  gas  is  passed  in  mitiute 
bubbles,  it  removes  with  it  a  certain  portion  of  the  chlorine.  From  this,  therefore,  it 
is  plain  that  muriatic  acid  thus  decomposed  will  not  yield  equal  measures  of  chlorine 
and  hydrogen,  unless  it  has  been  previously  impregnated  with  a  certain  volume  of  the 
former  gas.  Nor  is  it  possible  to  obtain  that  degree  of  saturation  by  voltaic  action,  no 
matter  how  long  the  electrolysis  is  continued,  if  the  hydrogen  is  allowed  to  pass  through 
the  liquid. 

868.  Practically,  therefore,  to  obtain  the  tithonometric  liquid,  we  are  obliged  to  de- 
compose commercial  muriatic  acid  in  a  glass  vessel,  the  positive  electrode  being  at  the 
bottom  of  the  vessel  and  the  negative  at  the  surface  of  the  liquid.  Under  these  circum- 
stances, the  chlorine,  as  it  is  disengaged,  is  rapidly  taken  up,  and  the  hydrogen  being 
set  free  without  its  bubbles  passing  through  the  mass,  the  impregnation  is  carried  to 
the  point  required. 

869.  Although  this  chlorinated  muriatic  acid  cannot,  of  course,  be  kept  in  contact 
with  the  platina  wires  without  acting  on  them,  the  action  is  much  slower  than  might 
have  been  anticipated.  I  have  examined  the  wires  of  tithonometers  that  have  been  in 
active  use  for  four  months  and  could  not  perceive  the  platina  sensibly  destroyed.    It  is 

B  B 


194 


THEORETICAL  CONDITIONS  OF  EQUILIBRIUM. 


well,  however,  to  put  a  piece  of  platina  foil  in  the  bottle  in  which  the  supply  of  chlori- 
nated muriatic  acid  is  kept :  it  communicates  to  it  slowly  the  proper  golden  tint. 

870.  The  liquid,  being  impregnated  with  chlorine  in  this  manner  until  it  exhales  the 
odour  of  that  gas,  is  to  be  transferred  to  the  siphon,  a  b  c,  oi  the  tithonometer,  and  its 
constitution  finally  adjusted  as  hereafter  shown. 

871.  Thirdly,  of  the  Voltaic  Battery. — The  battery  which  will  be  found  most  appli- 
cable for  these  purposes  consists  of  two  Grove's  cells,  the  zinc  surrounding  the  platina. 

872.  The  following  are  the  dimensions  of  the  pairs  which  I  use.  The  platina  plate 
is  half  an  inch  wide  and  two  inches  long ;  it  dips  into  a  cylinder  of  porous  biscuit- 
ware  of  the  same  dimensions,  which  contains  nitric  acid.  Outside  this  porous  vessel 
is  the  zinc,  which  is  a  cylinder  one  inch  diameter,  two  inches  long,  and  two  tenths 
thick  ;  it  is  amalgamated.  The  whole  is  contained  in  a  cup  two  inches  in  diameter 
and  two  deep,  which  receives  the  dilute  sulphuric  acid. 

873.  The  force  of  this  battery  is  abundantly  sufficient  both  for  preparing  the  fluid 
originally  and  for  carrying  on  the  tithonometric  operations ;  it  can  decompose  muriatic 
acid  with  rapidity,  and  will  last,  with  ordinary  care,  for  a  long  time. 

874.  Before  passing  to  the  mode  of  using  the  tithonometer,  it  is  absolutely  necessary 
to  understand  certain  theoretical  conditions  of  its  equilibrium ;  to  these  in  the  next 
place  I  shall  revert. 

875.  Theoretical  Conditions  of  Equilibrium. — The  tithonometer  depends  for  its  sen- 
sitiveness on  the  exact  proportion  of  the  mixed  gases.  If  either  one  or  the  other  is  in 
excess,  a  great  diminution  of  delicacy  is  the  result.  The  comparison  of  its  indications 
at  different  times  depends  on  the  certainty  of  evolving  the  gases  in  exact,  or,  at  all  events, 
known  proportions. 

876.  Whatever,  therefore,  affects  the  constitution  of  the  sentient  gases,  alters,  at  the 
same  time,  their  indications.  Between  those  gases  and  the  fluid  which  confines  them 
certain  relations  subsist,  the  nature  of  which  can  be  easily  traced.  Thus,  if  we  had 
equal  measures  of  chlorine  and  hydrogen,  and  the  liquid  not  saturated  with  the  former, 
it  would  be  impossible  to  keep  them  without  change,  for,  by  degrees,  a  portion  of  chlo- 
rine would  be  dissolved,  and  an  excess  of  hydrogen  remain  ;  or,  if  the  liquid  was  over- 
charged with  chlorine,  an  excess  of  that  gas  would  accumulate  in  the  sentient  tube. 

877.  It  is  absolutely  necessary,  therefore,  that  there  should  be  an  equilibrium  be- 
tween the  gaseous  mixture  and  the  confining  fluid. 

878.  As  has  been  said,  when  muriatic  acid  is  decomposed  by  a  voltaic  current,  all 
the  chlorine  is  absorbed  by  the  liquid,  and  accumulates  therein ;  the  hydrogen  bubbles, 
however,  as  they  rise,  withdraw  a  certain  proportion,  and  hence  pure  hydrogen  passed  up 
through  the  tithonometric  fluid  becomes  exceedingly  sensitive  to  the  light. 

879.  There  are  certain  circumstances  connected  with  the  constitution  and  use  of 
the  tithonometer  which  continually  tend  to  change  the  nature  of  its  liquid.  The  pla- 
tina wires  immersed  in  it,  by  slow  degrees,  give  rise  to  a  chloride  of  platina.  It  is  true 
that  this  takes  place  very  gradually,  and  by  far  the  most  formidable  difficulty  arises  from 
a  direct  exhalation  of  chlorine  from  the  narrow  tube  ef  for  each  time  that  the  liquid 
descends,  a  volume  of  air  is  introduced,  which  receives  a  certain  amount  of  chlorine, 


/ 

DIRECTIONS  FOR  USING  TUIZ  TITHONOMETER.  295 

which,  with  it,  is  expelled  the  next  time  the  battery  raises  the  column  to  zero  ;  and 
this,  going  on  time  after  time,  finally  impresses  a  marked  change  on  the  liquid.  I  have 
tried  to  correct  this  in  various  ways,  as  by  terminating  the  end  /  with  a  bulb ;  but  this 
entails  great  inconvenience,  as  may  be  discovered  hy  any  one  who  will  reflect  on  its 
operation. 

880.  When  by  the  battery  we  have  raised  the  index  to  its  zero  point,  if  the  gas  and 
liquid  are  not  in  equilibrio,  that  zero  is  liable  to  a  slight  change.  If  there  be  hydrogen 
in  excess,  the  zero  will  rise;  if  chlorine,  the  zero  will  fall. 

881.  In  making  what  will  be  termed  "  interrupted  experiments,"  we  must  not  too 
hastily  determine  the  position  of  the  index  on  the  scale  at  the  end  of  a  trial.  It  is  to 
be  remembered  that  the  cause  of  movement  over  the  scale  arises  from  a  condensation 
of  muriatic  acid,  but  that  condensation,  though  very  rapid,  is  not  instantaneous.  Where 
time  is  valuable,  and  the  instrument  in  perfect  equilibrium,  this  condensation  may  be 
instantaneously  effected  by  simply  inclining  the  instrument  so  that  its  liquid  may  pass 
down  to  the  closed  end  a,  but  not  so  much  as  to  allow  gas  to  escape  into  the  other 
leg;  the  inclination  of  the  two  legs  to  each  other  makes  this  a  very  easy  manipulation, 
and  the  gas,  thus  brought  into  contact  with  an  extensive  liquid  surface,  yields  up  its  mu- 
riatic acid  in  a  moment. 

882.  Directions  for  using  the  Tithonometer.  Preliminary  Adjustment. — Having 
transferred  the  liquid  to  the  sealed  end  of  the  siphon,  and  placed  the  cap  on  the  sen- 
tient extremity,  the  voltaic  battery  being  prepared,  the  operator  dips  its  polar  wires  into 
the  cupsp^-,  which  are  in  connexion  with  the  wires  xy.  Decomposition  immediate- 
ly takes  place,  chlorine  and  hydrogen  rising  through  the  liquid,  and  gradually  depress- 
ing it,  while,  of  course,  a  corresponding  elevation  takes  place  in  the  other  limb ;  this 
operation  is  continued  until  the  liquid  has  risen  to  the  zero.  It  takes  but  a  few  seconds 
for  this  to  be  accomplished. 

883.  The  polar  wires  having  been  disengaged,  the  tithonometer  is  removed  opposite 
a  window,  care  being  taken  that  the  light  is  not  too  strong.  The  cap  is  now  lifted  off 
the  sentient  extremity,  a  d,  and  immediately  the  liquid  descends.  This  exposure  is 
allowed  to  continue,  and  the  liquid  suffered  to  rise  as  much  as  it  will  to  the  end  a. 
And  now,  if  the  gases  have  been  properly  adjusted,  an  entire  condensation  will  take 
place,  the  sentient  tube  a  d  filling  completely.  In  practice,  this  precision  is  not,  how- 
ever, obtained ;  and  if  a  bubble  as  large  as  a  peppercorn  be  left,  the  operator  will  be 
abundantly  satisfied  with  the  sensitiveness  of  his  instrument.  Commonly,  at  first,  a 
large  residue  of  hydrogen  gas,  occupying,  perhaps,  an  inch  or  more,  will  be  left.  It  is 
to  be  understood  that  even  this  large  surplus  will  disappear  in  a  few  hours  by  absorb- 
ing chlorine.  But  this  is  not  to  be  waited  for :  as  soon  as  no  farther  rise  takes  place 
in  a  minute  or  two,  the  siphon  is  to  be  inclined  on  one  side,  and  the  residue  turned  out 
into  the  open  leg. 

884.  Now,  recurring  to  what  has  been  said  on  the  equilibrium,  it  is  plain  that  this 
excess  of  hydrogen  arises  from  a  want  of  chlorine  in  the  titlionometric  liquid.  A  proper 
quantity  must,  therefore,  be  furnished  by  proceeding  as  follows: 

885.  The  sentient  tube  being  filled  with  the  liquid  by  inchnation,  connect  the  polar 


196 


METHODS  OF  CONTINUOUS  AND  INTERRUPTED  OBSERVATION. 


wires  with  p  q,  as  before.  These  may  be  called  generating  wires.  Allow  the  liquid  to 
rise  m  h  c  until  the  third  platina  wire,  z,  which  may  be  called  the  adjusting  toire,  is 
covered  an  eighth  of  an  inch  deep.  Then  remove  the  negative  wire  from  the  cup  ^> 
into  the  cup  r,  and  now  the  conditions  for  saturating  the  liquid  are  complete ;  hydro- 
gen escaping  away  from  the  surface  of  the  liquid  at  z,  and  chlorine  continually  accu- 
mulating and  dissolving  between  x  and  d.  This  having  been  carried  on  for  a  short 
time,  the  gas  in  a  is  to  be  turned  out  by  inclination,  and  the  instrument  recharged. 
That  a  proper  quantity  is  evolved  is  easily  ascertained  by  allowing  total  condensation 
to  take  place,  and  observing  that  only  a  small  bubble  is  left  at  a. 

886.  It  will  occasionally  happen  in  this  preliminary  adjustment,  that  an  excess  of 
chlorine  may  arise  from  continuing  the  process  too  long.  This  is  easily  discovered  by 
its  greenish-yellow  tint,  and  is  to  be  removed  by  incUning  the  instrument  and  turning 
it  out. 

887.  Thus  adjusted,  everything  is  ready  to  obtain  measures  of  any  effect,  there  being 
two  different  methods  by  which  this  can  be  done:  1st,  by  continuous  observation;  2d, 
by  interrupted  observation. 

888.  Of  the  Method  of  Continuous  Ohservation. — This  is  best  described  by  resorting 
to  an  example.  Suppose,  therefore,  it  is  required  to  verify  Table  I.,  or,  in  other  words, 
to  prove  that  the  effect  on  the  tithonometer  is  proportioned  to  its  time  of  exposure. 

Put  on  the  cap  of  the  sentient  tube  a  d,  connect  the  polar  wires  with  p  q,  and  raise 
the  liquid  to  zero. 

Place  the  tithonometer  so  that  its  sentient  tube  will  receive  the  rays  properly. 

At  a  given  instant,  marked  by  a  seconds  watch,  remove  the  cap  A  D,  and  the  liquid 
at  once  begins  to  descend.  At  the  end  of  the  first  minute  read  off  the  division  over 
which  it  is  passing.  Suppose  it  is  7.  At  the  end  of  the  second  do  the  same,  it  should 
be  14;  at  the  end  of  the  third,  21,  &c.,  &c.  This  may  be  done  until  the  fiftieth  divis- 
ion is  reached,  which  is  the  terminus  of  the  scale. 

Recharge  the  tube  by  a  momentary  application  of  the  polar  wires :  but  it  is  conve- 
nient first  to  remove  any  excess  of  muriatic  acid  gas  in  the  sentient  tube,  by  allowing  it 
tiuie  for  condensation  ;  or,  if  that  be  inadmissible,  by  inclining  a  little  on  one  side,  so  as 
to  give  an  extensive  liquid  contact. 

889.  Of  the  Method  of  Interrupted  Ohservation. — It  frequently  happens  that  obser- 
vations cannot  be  had  during  a  continuous  descent,  as  when  changes  have  to  be  made 
in  parts  of  apparatus  or  arrangements.  We  have,  then,  to  resort  to  interrupted  obser- 
vations. 

This  method  requires  that  the  gas  and  liquid  should  be  well  adjusted,  so  that  no 
change  can  arise  in  volume  when  extensive  contact  is  made  by  inclination. 

The  tithonometer  being  charged,  place  it  in  a  proper  position.  At  a  given  instant 
remove  its  cap,  and  the  liquid  descends.  When  the  time  marked  by  a  seconds  watch 
has  elapsed,  drop  the  cap  on  the  sentient  tube.  The  liquid  simulianeously  pauses  in 
its  descent,  but  does  not  entirely  stop,  for  a  little  uncondensed  muriatic  acid  still  exists, 
which  is  slowly  disappearing  in  the  sentient  tube.  Now  incline  the  instrument  for  a 
moment  on  one  side,  so  that  the  liquid  may  run  up  to  the  end  a,  but  not  so  much  as  to 


REMARKABLE  CONTRACTION  AND  EXPANSION. 


197 


let  any  gas  escape.  Restore  it  to  its  position,  and  read  off  on  the  scale.  It  is  then 
ready  for  a  second  trial. 

890.  The  difference  between  continuous  and  interrupted  observation  is  this,  that  in 
the  latter  we  pause  to  wash  out  the  muriatic  acid ;  and  though  this  is  effected  by  the 
simplest  of  all  possible  methods,  continuous  observations  are  always  to  be  preferred 
when  they  can  be  obtained. 

891.  I  have  extended  this  paper  to  so  great  a  length  that  many  points  on  which  re- 
marks might  have  been  made  must  be  passed  over.  It  is  scarcely  necessary  to  say  that 
the  sentient  tube  must  be  uniformly  and  perfectly  clean.  As  a  general  rule,  also,  the 
first  observation  may  be  cast  aside,  for  reasons  which  I  will  give  hereafter.  Farther,  it 
is  to  be  remarked,  as  it  is  an  essential  principle  that  during  difK'erent  changes  of  volume 
of  the  gas  its  exposed  surface  must  never  vary  in  extent,  the  liquid  is  not  to  be  suffered 
to  rise  above  the  blackened  portion  at  d.  If  the  measures  of  the  different  parts  be  such 
as  have  been  here  given,  this  cannot  take  place,  for  the  liquid  will  fall  below  the  fiftieth 
division  before  its  other  extremity  rises  above  d. 

892.  The  same  original  volume  of  gas  va  a  d  will  last  for  a  long  time,  as  we  keep 
replenishing  it  as  often  as  the  fiftieth  division  is  reached. 

893.  The  experimenter  cannot  help  remarking,  that  on  suddenly  exposing  the  sen- 
tient tube  to  a  bright  light,  the  liquid  for  an  instant  rises  on  a  scale,  and  on  dropping  the 
cap,  in  an  instant  falls.  This  important  phenomenon,  which  is  strikingly  seen  under 
the  action  of  an  electric  spark,  I  shall  consider  hereafter. 

894.  In  conclusion,  as  to  comparing  the  tithonometric  indications  at  different  times, 
if  the  gases  have  the  same  constitution,  the  observations  will  compare  ;  and  if  they  have 
not,  the  value  can  from  time  to  time  be  ascertained  by  exposure  to  a  lamp  of  constant 
intensity.    To  this  method  I  commonly  resort. 

895.  From  the  space  occupied  in  this  description  the  reader  might  be  disposed  to 
infer  that  the  tithonometer  is  a  very  complicated  instrument,  and  difficult  to  use.  He 
would  form,  however,  an  erroneous  opinion.  The  preliminary  adjustment  can  be  made 
in  five  minutes,  and  with  it  an  extensive  series  of  measures  obtained.  These  long 
details  have  been  entered  into  that  the  theory  of  the  instrument  may  be  known,  and 
optical  artists  construct  it  without  difficulty.  Though  surprisingly  sensitive  to  the  ac- 
tion of  the  indigo  ray,  it  is  as  manageable  by  a  careful  experimenter  as  a  common  dif- 
ferential thermometer. 


198 


TITHONIZED  CHLORINE. 


CHAPTER  XVII. 

ON  TITHONIZED  CHLORINE. 

Contents  :  Description  of  the  Experiinent. —  The  Change  in  the  Chlorine  is  not 
Transient. —  There  are  two  Stages  in  the  Phenomenon. — Rays  are  ahsorhed  in 
producing  this  Change. — It  is  the  Indigo  Ray  which  is  ahsorhed. —  The  Action  is 
positive  from  End  to  End  of  the  Spectrum. —  The  Indigo  Ray  forms  Muriatic  Acid, 
as  well  as  produces  the  Preliminary  Tithonization. — Change  in  other  Elementary 
Bodies. —  Verification  of  the  preceding  Results  with  the  Tithonometer. 

(From  the  London,  Edinburgh,  and  Dublin  Philosophical  Magazine  fur  July,  1844.) 

896.  Chlorine  gas,  which  has  been  exposed  to  the  dayhght  or  to  sunshine,  possesses 
quaUties  which  are  not  possessed  by  chlorine  which  bas  been  made  in  the  dark. 

897.  This  is  shown  by  the  circumstance  that  chlorine  which  has  been  exposed  to 
the  sunshine  has  obtained  from  that  exposure  the  property  of  speedily  uniting  with  hy- 
drogen gas ;  a  property  not  possessed  by  chlorine  which  has  been  made  and  kept  in 
the  dark. 

898.  This  quality  gained  by  the  chlorine  arises  from  its  having  absorbed  tithonic 
rays  corresponding  in  refrangibility  to  the  indigo.  Ii  is  not  a  transient,  but  apparently 
a  permanent  property,  the  rays  so  absorbed  becoming  latent,  and  the  effect  lasting  for 
an  unknown  period  of  time.  The  facts  which  I  shall  proceed  to  describe  will  be  in- 
teresting to  chemists,  because  they  plainly  lead  us  to  suspect  that  the  descriptions  we 
have  of  the  properties  of  all  elementary  and  compound  bodies  are  either  inaccurate  or 
confused.  These  properties  are  such  as  bodies  exhibit  after  they  have  been  exposed 
to  the  light ;  we  still  require  to  know  what  are  the  properties  they  possess  before  ex- 
posure to  such  influences. 

899.  Natural  philosophers  will  also  find  an  interest  in  these  phenomena,  for  they 
finally  establish  for  the  tithonic  rays  two  important  facts :  1st,  that  those  rays  are  ab- 
sorbed by  ponderable  bodies  ;  and,  2d,  that  they  become  latent  after  tbe  manner  of  heat. 
Some  years  ago  I  endeavoured  to  prove  that  these  things  held  for  a  compound  sub- 
stance the  iodide  of  silver  {Phil.  Mag.,  Sept.,  1841). 

900.  For  reasons  which  will  be  obvious  as  the  description  proceeds,  I  shall  speak 
of  chlorine  which  has  been  exposed  to  the  beams  of  the  sun  as  tithonized  chlorine. 

I.  Description  of  the  Experiment. 

901.  In  two  similar  white  glass  tubes  place  equal  volumes  of  chlorine,  which  has 
been  made  from  peroxide  of  manganese  and  muriatic  acid  by  lamplight,  and  carefully 
screened  from  access  of  daylight.  Expose  one  of  the  tubes  to  the  full  sunbeams  for 
some  minutes,  or,  if  the  light  be  feeble,  for  a  quarter  of  an  hour :  the  chlorine  which  is 
in  it  becomes  tithonized.  Keep  the  other  tube  during  this  time  carefully  in  a  dark 
place ;  and  now,  by  lamphght,  add  to  both  equal  volumes  of  hydrogen  gas.  These 


/ 


DESCRIPTION  OF  THE  EXPERIMENT.  jgg 

processes  are  best  carried  on  in  a  small  porcelain  or  earthenware  trough,  filled  with  a 
saturated  solution  of  common  salt,  which  dissolves  chlorine  slowly ;  and  to  avoid  ex- 
plosions, operate  on  limited  quantities  of  the  gases.  Tubes  that  are  eight  inches  long 
and  half  an  inch  in  diameter  will  answer  very  well.  The  two  tubes  now  contain  the 
same  gaseous  mixture,  and  only  differ  in  the  circumstance  that  one  is  tithonized  and 
the  other  not.  Place  them,  therefore,  side  by  side  before  a  window,  through  which  the 
entrance  of  daylight  can  be  regulated  by  opening  the  shutter ;  and  now,  if  this  part  of 
the  process  is  conducted  properly,  it  will  be  seen  that  the  tithonized  chlorine  com- 
mences to  unite  with  the  hydrogen,  and  the  salt  water  rises  in  that  tube.  But  the  un- 
tithonized  chlorine  shows  no  disposition  to  unite  with  its  hydrogen,  and  the  liquid  in 
its  tube  remains  motionless  for  a  long  time.  Finally,  as  it  becomes  slowly  tithonized 
by  the  action  of  the  daylight  impinging  on  it,  union  at  last  takes  place.  From  this, 
therefore,  we  perceive  that  chlorine  which  has  been  exposed  to  the  sun  will  unite 
promptly  and  energetically  with  hydrogen ;  but  chlorine  that  has  been  made  and  kept 
in  the  dark  shows  no  such  property. 

902.  As  I  doubt  not  this  remarkable  experiment  will  be  repeated  by  chemists,  I  will 
add  that  the  only  point  to  which  attention  in  particular  is  to  be  given,  is  in  the  final 
exposure  to  the  light.  This  must  not  be  too  feeble,  or  the  action  will  be  tedious  ;  but 
the  direct  sunbeam  must  be  sedulously  excluded,  or  an  explosion  will  result.  A  room 
illuminated  by  one  small  window,  looking  to  the  north,  answers  very  well.  It  need 
scarcely  be  added,  that  care  must  be  taken  that  both  tubes  are  illuminated  alike. 

II.  TJte  Change  in  the  Chlorine  is  not  Transient. 

903.  Now  it  might  be  supposed  that  this  apparent  exaltation  of  the  electro-negative 
properties  of  the  chlorine  is  only  a  transient  thing  which  would  speedily  pass  away, 
the  gas  reverting  to  its  original  untithonized  condition. 

904.  To  show  that  this  is  not  so,  tithonize  some  chlorine  in  a  tube  as  before.  Place 
It  for  an  hour  or  two  in  the  dark  along  with  the  tube  of  untithonized  chlorine,  with 
which  it  is  to  be  compared  ;  then  to  both  add  hydrogen.  Expose  them,  as  in  the  for- 
mer experiment,  to  the  daylight,  and  the  result  will  turn  out  as  before,  the  tithonized 
chlorine  forming  muriatic  acid  at  once,  and  the  untithonized  refusing  to  do  so. 

905.  This,  therefore,  shows  that  the  change  which  the  sunbeams  impress  upon 
chlorine  is  to  a  certain  extent  a  permanent  change,  and,  unlike  a  calorific  effect,  it  does 
not  spontaneously  and  rapidly  pass  away. 

III.  There  are  two  Stages  in  the  Phenomenon. 

906.  Let  us  now  proceed  to  make  an  inquiry  into  the  nature  of  the  change  thus  im- 
pressed on  the  chlorine.  This,  I  shall  show,  rests  in  the  circumstance  of  the  absorption 
of  rays  which  correspond  in  refrangibility  to  the  indigo,  and  which  appear  to  become 
latent. 

907.  In  a  tube,  over  salt  water,  mix  together  equal  volumes  of  untithonized  chlorine 
and  hydrogen  gas.  Expose  it  to  the  daylight,  marking  the  time  at  which  the  exposure 
commences.    Watck  the  level  of  the  liquid  in  the  tube  narrowly,  and,  though  station- 


RAYS  ARE  ABSORBED. 


ary  for  a  considerable  time,  after  a  certain  period  has  elapsed  it  will  be  seen  on  a  sud- 
den to  start  and  commence  rising.  Observe  now  how  far  it  will  rise  during  a  period 
which  is  equal  to  the  time  that  elapsed  between  the  first  exposure  and  the  beginning 
of  the  rise,  and  it  will  be  seen  that  one  fourth  or  half  of  the  gases  will  disappear. 

908.  It  is  obvious  that  from  the  first  moment  of  exposure  the  rays  must  have  been 
exerting  their  influences  on  the  mixture.  As  will  presently  be  proved,  absorption  has 
been  all  along  taking  place.  There  are,  therefore,  two  distinct  phenomena  exhibited 
by  this  experiment.  There  is  a  period  during  which,  though  large  quantities  of  the 
dark  rays  are  disappearing,  no  visible  change  is  produced ;  there  is  a  second  period, 
during  which  absorption  is  accompanied  by  a  remarkable  chemical  effect,  the  produc- 
tion of  muriatic  acid.  From  these  things  we  gather  that  a  definite  amount  of  the 
tithonic  rays  must  disappear  and  become  latent  before  muriatic  acid  can  form.  The 
phenomenon  is  not  unlike  that  of  the  disappearance  of  a  definite  quantity  of  heat  in  the 
passage  of  ice  into  the  condition  of  water. 

909.  A  mixture  of  chlorine  and  hydrogen  does  not,  therefore,  instantly  give  rise  to 
the  production  of  muriatic  acid  on  exposure  to  the  light,  but,  as  a  preliminary  condition, 
a  certain  definite  amount  of  absorption  must  take  place. 

910.  Now  if  this  were  a  mere  molecular  disturbance,  such  as  might  be  brought  about 
by  the  action  of  heat,  we  should  expect  to  find  it  transient  and  speedily  passing  away. 
Such,  however,  is  far  from  being  the  case.  As  with  simple  chlorine,  so  with  this  mix- 
ture, after  it  has  been  tithonized  it  loses  its  quality  very  slowly.  I  have  observed  that 
after  a  week  or  more  has  elapsed  since  it  was  first  exposed  to  the  light,  it  commences 
to  contract  when  placed  in  a  feeble  gleam. 

IV.  Rays  are  absorbed  in  producing  this  Change. 

911.  I  have  thus  far  assumed  that  the  rays  which  bring  about  these  are  absorbed; 
the  following  is  the  proof  which  I  have  to  offer : 

912.  Over  a  tube  half  an  inch  in  diameter  and  six  inches  long,  closed  at  its  upper 
extremity  and  open  at  its  lower,  invert  a  jar  of  the  same  length  and  one  inch  and  a 
half  in  diameter.  Fill  the  tube  and  the  jar  at  the  salt  water  trough,  about  two  thirds 
full,  with  the  same  mixture  of  chlorine  and  hydrogen.  Expose  them  to  diffuse  day- 
light. Now  it  is  clear  that  no  rays  can  gain  access  to  the  tube,  except  after  having 
passed  through  the  gaseous  mixture  in  the  jar.  After  a  certain  space  of  time  the  level 
of  the  liquid  in  the  jar  commences  to  rise,  but  that  in  the  tube  will  remain  much  longer 
wholly  stationary. 

913.  It  therefore  appears  that  a  beam  which  has  passed  through  a  mixture  of  chlorme 
and  hydrogen  has  lost,  to  a  great  extent,  the  quality  of  bringing  about  the  union  of  a 
second  portion  of  the  mixed  gases  through  which  it  may  be  caused  to  traverse.  The 
active  rays  have  been  absorbed  ;  they  disappear  from  the  beam,  and  are  lost  in  producing 
their  first  effect. 

914.  A  beam  of  light  becomes  detithonized  in  producing  a  chemical  effect ;  the 
beam,  as  well  as  the  medium  on  which  it  acts,  becomes  changed.  I  have  a  series  of 
results  which  proves  that  this  takes  place  for  a  great  variety  of  compound  bodies. 


THE  INDIGO  RAY  IS  ABSORBED. 


201 


V.  It  is  the  Indigo  Ray  which  is  absorbed. 

915.  As  has  been  said,  it  is  a  ray  which  corresponds  in  refrangibility  to  the  indigo 
which  produces  these  results. 

916.  In  a  small  porcelain  trough  I  inverted,  side  by  side,  ten  tubes,  each  of  which 
was  three  inches  long  and  one  third  of  an  inch  in  diameter,  the  trough  being  filled  with 
salt  water.  I  passed  into  each  tube  a  certain  quantity  of  untithonized  chlorine  and 
hydrogen.  A  beam  of  the  sun,  being  directed  by  a  heliostat  into  a  dark  room,  was 
dispersed  horizontally  by  a  flint  glass  prism,  and  the  trough  with  its  tubes  so  placed  as 
to  offer  an  exposure  to  the  different  coloured  rays.  The  aperture  which  admitted  the 
beam  was  about  half  an  inch  in  diameter.  For  a  while  no  niovement  was  observed  in 
any  of  the  tubes ;  but  as  soon  as  the  preliminary  absorption,  previously  described,  was 
over,  and  tlie  tithonization  completed,  the  level  of  the  liquid  began  to  rise.  In  the  red 
and  in  the  orange  no  movement  could  be  perceived,  in  the  violet  only  after  a  time ;  but 
first  of  all  the  tube  that  was  immersed  in  the  indigo  light  was  in  action,  and  exhibited 
finally  a  very  rapid  rise ;  this  was  soou  followed  by  the  tube  that  was  in  the  space 
where  the  indigo  and  violet  joined,  then  by  that  in  the  violet,  and  that  in  the  blue ; 
the  tube  in  the  green  was  next  in  order.  The  following  table  gives  the  numerical  re- 
sults obtained  by  observing  the  time  which  elapsed  before  movement  took  place  in 
each  tube  : 


TABLE  I. 


Name  of  Ray. 

Time. 

Name  of  Ray. 

Time. 

Extreme  red  .... 
Red  and  orange 
Yellow  and  green 
Green  and  blue 

Blue  

-1-100  00 
.52  00 
4  00 
233 

Indigo  ..... 
Indigo  and  violet 

Violet  

Violet  

Extreme  violet 

1  50 

2  00 
2  25 
5  00 
5  50 

917.  Many  years  ago,  M.  Berard  made  experiments  on  the  explosion  of  chlorine  and 
hydrogen,  and  concluded,  from  his  results,  that  it  was  brought  about  by  the  violet  ray. 
This  was  at  a  time  when  the  methods  of  making  these  experiments  were  less  exactly 
known.  It  is  a  very  easy  matter  to  prove  that,  in  reality,  the  indigo  is  the  active  ray, 
and  that,  from  a  maximum  point  which  is  in  the  indigo,  but  towards  the  blue,  the  effect 
gradually  diminishes  to  each  end  of  the  spectrum. 

918.  The  following  table  gives  the  calculated  approximate  intensity  of  the  chemical 
force  for  each  ray,  deduced  from  the  foregoing  experiment : 


TABLE  II. 


Name  of  Ray. 

Force. 

Name  of  Ray. 

Force. 

Extreme  red  .... 

Indigo  ..... 

66  60 

Red  and  orange 

100 

Indigo  and  violet 

50  00 

Yellow  and  green 

1  90 

Violet  ..... 

44  40 

Green  and  blue 

25  00 

Violet  

20  00 

Blue  

4290 

Extreme  violet 

18  10 

919.  There  is  a  great  advantage  which  experiments  conducted  in  this  way  possess 
over  those  which  depend  for  their  indication  on  the  stains  impressed  on  Daguerreotype 
plates  or  sensitive  papers.  In  those  cases  we  obtain  merely  a  comparative  contrast  for 
different  regions  of  the  spectrum  ;  in  this  we  have  absolute  measures  determined  by  a 

*  Even  after  the  longest  exposure  I  had  the  means  of  giving  it,  no  movement  took  place  in  the  tube  which  was  in  the 
extreme  red,  and  I  am  doubtful  about  that  in  the  red  and  orange. 


202 


THE  ACTION  IS  POSITIVE  THROUGH  THE  SPECTRUM. 


definite  chemical  effect,  and  the  rise  of  a  hquid  in  a  graduated  tube ;  and  from  this  we 
gain  juster  views  of  the  true  constitution  of  the  spectrum.  On  studying  the  numbers 
in  the  foregoing  table,  or  better  still,  if  we  project  them,  it  will  appear  what  an  enor- 
mous difference  there  is  in  the  chemical  force  of  the  different  rajs.  In  the  exj)eriment 
from  which  I  have  deduced  this  table,  it  appears  that  the  force  of  the  indigo  raj  ex- 
ceeds that  of  the  orange  in  a  greater  ratio  than  C6  to  1 ;  and  from  the  circumstances 
under  which  the  experiment  is  made,  this  difference  must  be  greatly  underrated.  There 
is  always  diffused  light  in  the  room  coming  from  the  intromitted  beam,  and  this  accel- 
erates the  rise  in  the  less  refrangible  tubes ;  then,  again,  it  is  impossible  that  the  tube 
which  gives  the  greatest  elevation  shall  coincide  mathematically  with  the  maximum 
point  and  express  the  maximum  effect. 

920.  From  some  estimates  I  have  made,  I  am  led  to  believe  that,  in  point  of  chem- 
cal  force,  for  this  mixture  of  chlorine  and  hydrogen,  the  indigo  ray  exceeds  the  red 
in  a  higer  ratio  than  500  to  1. 

VI.  The  Action  is  positive  from  End  to  End  of  the  Spectrum. 

921.  M.  Becquerel  found  that  for  a  Daguerreotype  plate,  the  red,  the  orange,  and 
the  yellow  rays  possess  the  quality  of  continuing  the  action  begun  by  the  more  refran- 
gible colours ;  he  therefore  names  these  "  rayons  continuateursr  For  the  same  com- 
pound I  found  that  those  rays,  acting  conjointly  with  the  diffused  daylight,  exerted  a 
negative  agency.  It  is  therefore  desirable  to  understand  whether,  with  respect  to  the 
gases  now  under  consideration,  the  lesser  refrangible  rays  exert  anything  in  the  way 
of  an  action  of  depression  or  hinderance  to  union.  By  direct  experiment,  I  found  that 
this  was  not  the  case,  the  action  being  positive  from  end  to  end  of  the  spectrum.  This 
can  be  shown  by  removing  the  tubes,  after  they  have  been  in  the  spectrum  for  an 
hour  or  two,  into  the  gleams  of  daylight.  One  by  one  they  exhibit  after  a  time  a  rise, 
the  order  being  the  green  first,  then  the  yellow  and  the  orange,  and  at  last  the  red. 
And  if,  at  the  same  time,  a  tube  which  has  been  kept  in  the  dark  be  exposed  along 
with  them,  they  will  all  rise  before  it,  showing  that  tithonization  had  set  in  and  been 
going  on  in  them  all ;  that  it  had  been  more  active  in  the  green  than  in  the  yellow,  in 
the  yellow  than  in  the  orange,  in  the  orange  than  in  the  red  ;  and  had  the  exposure  to 
the  spectrum  been  long  enough,  the  liquid  in  every  one  of  the  tubes  would  have  risen. 

VII,  The  Indigo  Ray  forms  the  Muriatic  Acid,  as  well  as  produces  the  preliminary 

Tithonization. 

922.  It  only  now  remains  to  inquire,  whether  the  rays  which  cause  the  production 
of  the  muriatic  acid  are  those  which  effect  the  tithonization  of  ihe  chlorine ;  in  other 
words,  whether  the  first  stage  of  the  process  is  brought  about  by  the  same  agent  which 
carries  on  the  second.  The  experiment  which  I  have  just  described  shows  that 
tithonization  is  most  actively  produced  by  the  indigo  ray,  and  it  is  easy  to  show  that  it 
is  the  same  ray  which  carries  on  the  second  part  of  the  process ;  for  if,  before  placing 
the  tubes  in  the  prismatic  spectrum,  we  tithonize  them  in  the  dayUght,  so  that  the  liquid 
has  just  commenced  to  rise  in  each,  and  then  expose  them  to  the  spectrum,  it  will  be 


THE  CHANGE  IS  NOT  TRANSIENT.  203 

found  that  the  tube  in  the  indigo  rises  most  rapidly,  and  the  others  in  the  order  stated 
before.  Therefore  we  perceive  that  the  same  ray  commences,  carries  on,  and  com- 
pletes the  process. 

923.  Few  substances  can  exceed  in  sensitiveness  to  hght  a  mixture  of  chlorine  and 
hydrogen  previously  tithonized.  Brought  into  the  obscure  daylight  of  a  gloomy  cham- 
ber, it  is  remarkable  how  promptly  the  level  of  the  liquid  in  the  tube  rises ;  how,  when 
the  shutters  are  successively  thrown  open,  the  action  becomes  more  and  more  ener- 
getic ;  and  how,  in  an  instant,  it  stops  when  the  instrument  is  shaded  by  a  screen. 

924.  I  have  not  recorded  in  this  communication  a  multitude  of  experiments  of  de- 
tail, which  go  to  support  the  conclusions  here  drawn,  and  which  will  be  published  at  a 
proper  time.  It  has  been  my  object  on  this  occasion  to  call  attention  to  the  fact,  that 
chlorine,  an  elementary  body,  undergoes  a  change  after  exposure  to  the  light ;  a  change 
which  appears  to  produce  an  exaltation  of  its  electro-negative  properties,  as  is  shown 
by  its  power  of  uniting  more  energetically  with  hydrogen.  This  change  must  not  be 
confounded  with  those  transient  elevations  of  activity  due  to  increased  temperature, 
inasmuch  as  this  is  more  permanent  in  its  character.  It  arises  from  the  absorption  of 
rays,  which  exist  most  abundantly  in  the  indigo  space  of  the  spectrum.  That  the 
phenomenon  is  due  to  a  true  absorption,  is  fully  shown  in  the  circumstance,  that  a 
beam  which  has  produced  this  effect  has  lost  the  quality  of  ever  after  producing  a 
similar  result.  This  is  borne  out  by  what  we  observe  to  take  place  when  a  feeble 
light  falls  on  a  mixture  of  chlorine  and  hydrogen  which  has  been  prepared  in  the  dark. 
A  certain  space  of  time  elapses  before  any  formation  of  muriatic  acid  occurs,  during 
which  the  absorption  in  question  is  going  on ;  and  when  that  is  completed,  and  the 
mixture  is  tithonized,  union  of  the  gases  begins,  and  muriatic  acid  forms.  From  end 
to  end  of  the  spectrum  the  action  is  positive,  and  differs  only  in  intensity  ;  but  this 
difference  in  intensity  opens  before  us  new  views  of  the  constitution  and  character  of 
the  solar  beam. 

University  of  New- York,  June  20,  1843. 

925.  The  foregoing  paper  was  written  almost  a  year  ago,  and  since  that  time  I  have 
made  several  new  observations  corroborative  of  the  results  given. 

926.  Chlorine  is  not  the  only  elementary  substance  in  which  the  rays  produce  a 
change.  In  his  chapter  on  phosphorus,  Berzelius  remarks,  "  Light  produces  in  it 
(phosphorus)  a  peculiar  change,  of  which  the  intimate  nature  is  unknown  ;  and  which, 
so  far  as  we  can  judge  at  present,  does  not  alter  its  weight.  It  makes  it  take  a  red 
tint.  This  phenomenon  occurs  not  only  in  a  vacuum,  even  in  that  of  a  barometer,  but 
also  in  nitrogen  gas,  in  carburetted  hydrogen,  under  water,  alcohol,  oil,  and  other 
liquids.  When  we  expose  to  the  sunlight  phosphorus  dissolved  in  ether,  oil,  or  hy- 
drogen gas,  it  instantly  separates  under  the  form  of  red  phosphorus;  it  undergoes  very 
rapidly  this  modification  in  violet  light,  or  in  glass  vessels  of  a  violet  colour.  The 
light  of  the  sun  makes  it  easily  enter  into  fusion  in  nitrogen  gas,  but  it  do  s  not  melt 
in  hydrogen,  and  in  the  Torricellian  vacuum  it  sublimes  in  the  form  of  brilliant  red 
scales." — (Berzelius,  Traite,  tom.  i.,  p.  258.) 


204 


ANALOGOUS  CHANGE  IN  PHOSPHORUS. 


927.  Again,  when  speaking  of  pliosphuretted  hydrogen,  he  says,  "  Exposed  to  the 
influences  of  the  direct  solar  hght  tliis  gas  is  decomposed,  a  part  of  the  phosphorus 
separates  under  the  form  of  red  phosphorus,  and  is  deposited  on  the  interior  surl'ace  of 
the  glass.  If  we  cover  the  vessel  which  contains  the  gas  imperlectlj,  no  phosphorus 
is  deposited  on  the  covered  spaces." — (i/;.,  torn,  i.,  p.  265.) 

928.  As  Berzelius  does  not  give  these  experiments  as  his  own,  and  I  do  not  know 
to  whom  we  are  indebted  for  them,  I  repeated  some  of  them.  Among  other  corrobo- 
rative results,  it  appeared  that  a  piece  of  phosphorus  of  a  pale  or  whitish  colour,  in  a 
vessel  filled  with  pure  and  dry  carbonic  acid  gas,  placed  in  the  sunshine,  exhibited  the 
phenomenon  in  question.  Eventually  the  phosphorus  became  of  a  deep  blood-red 
colour,  and  on  the  sides  of  the  glass  towards  the  light  feathery  crystals  formed,  the  tint 
of  which  bore  a  close  resemblance  to  that  of  the  red  prussiate  of  potash. 

929.  Since  the  invention  of  the  tithonometer,  I  have  been  able  to  observe  more 
closely  the  habitudes  of  chlorine.  In  the  description  given  of  that  instrument  in 
Chapter  XVL,  it  is  recomn)ended  to  cast  aside  the  first  observation,  because  it  never 
gives  an  accurate  estimate  of  the  true  effect.  When  a  mixture  of  chlorine  and  hy- 
drogen is  exposed,  muriatic  acid  does  not  innnediately  form  ;  but  a  preliminary  titho- 
nization  is  necessary,  and  then,  at  the  end  of  a  certain  period,  contraction  begins  to 
take  place. 

930.  A  tithonometer  exposed  to  the  daylight  is  much  too  powerfully  affected  to 
allow  of  the  successive  stages  of  change  to  be  distinctly  made  out ;  the  preliminary 
tithonization  is  accomplished  so  rapidly,  that  the  indications  of  it  are  merged  and  lost 
in  the  contraction  which  instantly  follows.  It  is  necessary,  therefore,  that  we  should 
operate  with  a  small  lamp-flame. 

931.  To  such  a  flame  I  exposed  a  mixture  of  chlorine  and  hydrogen,  and  marked 
the  number  of  seconds  which  elapsed  before  contraction,  arising  from  the  production 
of  muriatic  acid,  took  place.  The  first  indications  of  movement  occurred  at  the  close 
of  600  seconds. 

The  index  then  moved  through  the  first  degree  in  480  seconds. 
"  "  "         second     "  165 

"  «  "         third        "  130 

«  «  "         fourth      "       95  " 

fifth         "       93  " 
"  "  "         sixth       "       93  " 

and  continued  to  move  with  regularity  at  the  same  rate. 

932.  These  observations,  therefore,  prove  that  a  very  large  amount  of  radiant  matter 
is  absorbed  before  chemical  combination  takes  place ;  and  that  in  the  case  of  chlorine 
and  hydrogen,  the  total  action  is  divisible  into  two  periods :  the  first,  during  which  a 
simple  absorption  is  taking  place  without  a  chemical  effect ;  the  second,  during  which 
absorption  is  attended  with  the  production  of  muriatic  acid. 

933.  The  facts  which  I  am  endeavouring  to  set  forth  prominently  in  this  communi- 
cation are,  1st,  the  preliminary  tithonization  just  discussed  ;  and,  2d,  the  persistent 
character  of  the  change  impressed  upon  chlorine  when  it  has  been  exposed  to  the  sun, 
an  effect  wholly  unlike  a  calorific  effect,  which  would  soon  disappear. 


/ 


A  FOURTH  IMPONDKRABLE..  205 

934.  By  resorting  to  the  tithonometer,  we  obtain  information  equally  distinct  upon 
the  second  point,  that  the  preliminary  tithonization  is  not  a  transient  effect  which  at 
once  passes  away,  but  is,  on  the  contrary,  a  persistent  change. 

935.  I  tithonized  the  chlorine  and  hydrogen  contained  in  the  instrument,  and  kept 
it  in  the  dark  for  ten  hours.  On  exposure  to  the  lamp-rays  it  moved  after  a  few  sec- 
onds, showing,  therefore,  that  the  change  which  had  been  impressed  on  the  chlorine 
was  not  lost.  In  the  former  case,  600  seconds  had  elapsed  before  any  movement  was 
visible. 

936.  When,  however,  we  remember  that  the  invisible  images  on  Daguerreotype 
plates,  and  even  photographic  impressions  on  surfaces  of  resin,  and  probably  all  other 
similar  changes,  are  slowly  effaced,  it  would  be  premature  to  conclude  that  tithonized 
chlorine  does  not  revert  to  its  original  condition.  I  have  sometimes  thought  that  there 
were  in  several  of  my  experiments  indications  that  this  was  taking  place,  but  would  not 
be  understood  to  assert  it  positively.  Whether  it  be  so  or  not,  one  thing  is  certain, 
that  the  taking  on  of  this  condition  and  the  loss  of  it  is  a  very  different  affair  from 
any  transient  exaltation  of  action  due  to  a  temporary  elevation  of  temperature,  or  the 
contrary  effect  produced  by  cooling. 


CHAPTER  XVIII. 

FARTHER  CONSIDERATIONS  ON  THE  EXISTENCE  OF  A  FOURTH  IMPONDERABLE. 

Contents  :  Defects  of  former  Evidence. — A  neio  Photometer. — Measures  of  the  Light 
transmitted  hy  Coloured  Solutions. — Explosion  of  Chlorine  and  Hydrogen  by  a  dis- 
tant Electric  Spark. — Abso)-ptive  Action  of  Media. 

The  Absorptive  Action  on  Light  and  the  Tithonic  Rays  follows  different  Latcs. 

Opacity  of  Glass  for  Phosphoric  Rays. — Determination  of  the  Refran gibility  of  the 
Phosphoric  Rays  of  an  Electric  Spark. — Refran  gibility  of  the  same  Rays  in  the  Vol- 
taic Arc  of  Flame. — Professor  Henry's  Experiments. 

These  Facts  serve  to  prove  that  there  are  more  than  three  Lnponderahles. 

(From  the  Londcm,  Edinburgh,  and  Dublin  Philosophical  Magazine  for  August,  1844.) 

937.  In  the  Philosophical  Magazine  for  December,  1842,  I  brought  forward  several 
facts  which  had  caused  me  to  form  the  opinion  that  the  chemical  rays  of  the  older  optical 
writers  constitute,  in  reality,  a  new  imponderable  substance,  which  should  be  placed  in 
the  same  rank  with  light,  heat,  and  electricity.  To  the  views  then  given,  I  propose,  in 
this  communication  to  return  again,  and  furnish  farther  proof  of  their  correctness.  An 
extended  examination,  which  has  occupied  me  several  years,  has  served  to  deepen  my 
conviction  of  the  truth  of  this  doctrine. 

938.  Great  changes  in  the  fundamental  theories  of  science  ought  not  to  be  ligliily 


206 


DEFECTS  OF  FORMER  EVIDENCE. 


admitted.  It  was  only  after  many  years  of  discussion,  and  multitudes  of  experiments, 
that  the  doctrine  of  the  unity  of  air  was  destroyed,  and  the  theory  of  the  intrinsic  dif- 
ferences of  gaseous  hodies  received.  This  was  unquestionably  the  most  important  event 
that  ever  happened  to  chemistry. 

939.  The  imponderable  principles  are  the  true  living  forces  of  chemistry.  The 
circumstance  that  they  do  not  exhibit  the  property  of  weight  is  only  an  incidental  af- 
fair, and  ought  never  to  have  been  regarded  as  their  leading  characteristic :  they  are 
the  regulating  forces  by  which  ponderable  matter  is  arranged  and  grouped.  If,  then, 
so  great  a  change  occurred  in  chemistry,  on  more  exact  views  being  obtained  of  its 
pneumatic  department,  what  may  not  be  expected  from  the  discussions  which  are  ari- 
sing on  the  nature  of  its  great  controlling  forces ! 

i)40.  There  is  another  point  of  view  from  which  these  investigations  assume  a 
deep  interest.  I  have  shown  in  Chapter  XV.  that,  by  resorting  to  prismatic  analysis 
in  physiological  researches,  very  remarkable  truths  appear.  The  function  of  diges- 
tion, which  is  carried  on  during  sunshine  by  the  leaves  of  plants,  is  under  the  control 
of  the  yellow  ray.  It  is  this  which  causes  the  decomposition  of  carbonic  acid,  furnishes 
solid  food,  and  gives  the  green  colour.  In  animals  similar  results  are  produced  by  the 
agency  of  a  nervous  system  ;  and  not  only  so,  but  all  the  various  operations  connected 
with  life  are  conducted  in  the  same  way.  There  is  one  class  of  nerves  which  gives 
action  to  the  respiratory  apparatus,  and  another  which  controls  digestion.  There  is 
one  class  which  presides  over  motion,  another  which  is  the  recipient  of  sensation,  a 
third  which  originates  all  the  processes  of  thought  and  intellectuality.  In  the  vegeta- 
ble world  the  same  idea  is  preserved,  developed,  perhaps,  in  a  less  elaborate  way,  but 
under  the  guidance  of  a  principle  equally  ethereal  and  refined.  The  beams  of  the  sun 
are  the  true  nervous  principle  of  plants.  To  the  yellow  ray  is  assigned  their  nutri- 
tive processes,  to  the  blue,  their  movements.  We  can,  therefore,  easily  understand  how 
it  is  that  botanists,  who  have  sought  in  the  interior  of  plants  for  indications  of  a  ner- 
vous agent,  have  never  found  them.    That  agent  is  external. 

941.  By  the  experiments  that  have  been  made  for  determining  the  nature  of  the 
chemical  radiations,  the  question  has  been  brought  down  to  very  narrow  limits.  There 
is  no  author  who  regards  them  as  connected  in  any  way  with  radiant  heat,  nor  any, 
except  the  wildest  speculator,  who  traces  them  to  electricity.  The  difficulty  is,  to  offer 
clear  and  undoubted  proof  that  they  are  distinct  from  light.  Herschel  has  directly 
admitted  this  distinction,  and  brought  forward  several  experiments  (jPhil.  Trans.,  1840, 
p.  38,  &c.)  in  support  of  this  view.  I  have  given  some  evidence  of  the  kind,  both  re- 
cently and  also  several  years  ago  (1837).  All  these  experiments  depend  on  a  com- 
parison of  tithonographic  stains  produced  by  solar  spectra  that  have  undergone  the  ac- 
tion of  absorptive  media,  and  the  effect  of  those  spectra  on  the  organ  of  vision  ;  a  com- 
parison, in  short,  of  different  visil)le  spectra  and  their  tithonographic  impressions. 

942.  From  this  comparison,  we  endeavour  to  prove  that  invisible  rays  may  be  iso- 
lated in  any  part  of  the  spectrum,  and,  if  invisible,  we  argue  that  they  are  not  light. 

943.  It  cannot  be  concealed,  however,  that  there  is  a  certain  degree  of  imperfection 
in  this  species  of  evidence :  an  accurate  conclusion  as  to  the  presence  or  absence,  or 


A.  NEW  PHOTOMETER. 


207 


quantity  of  light,  is  by  no  means,  under  these  circumstances,  an  easy  affair.  In  these 
distorted  spectra,  as  in  the  natural  one,  the  terminations  shade  off  gradually,  and  it  is 
difficult  to  say  where  the  light  in  reality  ends.  On  these  terminations  also,  where  the 
light  is  so  dilute  and  feeble,  tithonographic  action,  although  faint,  may,  by  prolonged 
exposure,  become  not  only  perceptible,  but  even  prominent.  In  tithonographic  action, 
time  enters  as  an  element ;  in  the  act  of  vision  it  does  not.  A  feeble  gleam  does  not 
become  more  bright  by  constantly  looking  at  it,  but  a  sensitive  surface  exposed  to  such 
a  gleam  is  more  and  more  affected  as  the  time  is  increased. 

944.  Considerations  like  these  demonstrate  the  necessity  of  investigating  the  ques- 
tion in  other  ways,  and  more  especially  since  M.  Becquerel,  one  of  the  ablest  writers 
on  these  matters,  has  undertaken  to  support  a  doctrine  which  denies  the  existence  of 
the  chemical  rays,  and  imputes  the  whole  action  to  light. — {Taylor s  Scientific  Memoirs, 
vol.  iii.,  part  12.) 

945.  This  doctrine,  however,  will  not,  I  am  persuaded,  stand  the  test  of  criticism : 
there  are  facts,  and  these  very  imposing  ones,  which  make  it  utterly  untenable.  It  is 
my  object,  in  this  paper,  to  set  forth  that  evidence,  and  offer  farther  and  clearer  proof 
of  the  physical  independence  of  the  tithonic  rays  and  light,  and  indirectly  establish  the 
existence  of  a  new  imponderable. 

946.  The  true  issue  of  the  question,  as  has  been  said,  rests  in  proving  a  clear  dis- 
tinction between  light  and  the  tithonic  rays ;  the  other  imponderables  may  be  left  out 
of  the  argument.  The  mechanical  properties  of  the  two  agents  are  so  closely  alike — 
reflexion,  refraction,  polarization,  interference,  &c. — taking  place  under  the  same  laws 
for  both,  that  the  discussion  necessarily  becomes  one  of  quantity  and  measure.  Will 
a  given  ray  of  light,  disturbed  by  the  action  of  absorptive  media,  change  its  luminous 
and  chemical  relations  i)ari  passu  t  or  can  we  alter  the  one  and  leave  the  other  un- 
touched \  Or,  changing  both  by  any  process  of  treatment,  do  both  change  to  the  same 
extent  \ 

947.  The  final  decision  of  this  question  obviously  rests  in  obtaining  accurate  meas- 
ures for  the  rays  of  light  and  for  the  tithonic  rays.  It  is  the  comparison  of  those  meas- 
ures which  is  to  settle  the  point. 

948.  In  Chapter  XVI.  I  have  described  an  instrument,  under  the  name  of  the  titho- 
nometer,  which  gives  indications  by  the  production  of  muriatic  acid  from  the  union 
of  chlorine  and  hydrogen.  This  instrument  is  affected  chiefly  by  the  indigo  rays,  or, 
more  correctly  speaking,  by  those  rays  which  extend  over  the  blue,  indigo,  and  violet 
spaces  of  the  spectrum,  having  their  maximum  in  the  indigo.  It  is  iniportant  that  the 
reader  should  keep  this  fact  in  mind. 

949.  Optical  writers  have  been  greatly  embarrassed  for  want  of  a  photometrical  in- 
strument which  can  measure  the  intensity  of  light;  the  chief  difficulty  in  the  way  is  the 
impossibility  of  contrasting  together  lights  that  differ  in  colour.  By  all,  it  is  admitted 
that  the  eye  is  able  to  judge  of  the  amount  of  illumination  of  white  surfaces,  or  the 
depth  of  shadows  within  small  limits  of  error,  provided  the  rays  compared  are  nearly 
of  the  same  tint. 

950.  But  in  the  discussion  on  which  I  am  now  entering,  this  very  difficulty  is  in- 


208 


A  NEW  PHOTOMETER. 


creased  a  hundred-fold.  We  are  required  to  measure  the  intensity  of  light  which  has 
passed  through  all  sorts  of  absorbent  media,  and,  therefore,  has  become  excessively 
coloured.  How  shall  we  compare  together  the  rays  which  have  gone  through  sulpho- 
cyanate  of  iron,  and  are  of  a  deep  blood-red,  with  those  that  have  passed  through  sul- 
phate of  copper,  and  are  of  a  bright  blue  ? 

951.  Nevertheless,  this  problem  is  capable  of  a  complete  solution,  and  a  photometer 
can  be  obtained  which  gives  results  comparable  with  those  of  the  tithonometer :  such 
an  instrument  I  have  constructed ;  it  is  exceedingly  simple,  as  the  following  considera- 
tions prove. 

952.  We  are  to  remember  that  the  tithonometer  gives  indications  which  are  expres- 
sive of  the  intensity  of  the  blue  rays  generally  ;  the  blue  tithonic  rays  are  the  rays  which 
it  measures.  In  using  the  term  blue,  it  will  be  understood  to  comprehend  the  blue,  in- 
digo, and  violet,  or  the  more  refrangible  rays  generally.  It  is  obvious,  therefore,  that 
the  photometer  which  is  to  be  used  with  it  must  measure  the  same  blue  rays,  or,  in 
other  words,  the  tithonometer  and  the  photometer  must  be  affected  by  rays  compre- 
hended between  the  same  limits  of  refrangibility. 

953.  This  can  be  effected  by  interposing  in  the  photometer  some  absorbent  medium, 
which  will  admit  no  rays  to  pass  it  except  such  as  are  in  the  limits  of  refrangibility 
with  which  the  tithonometer  is  engaged.  It  is  fortunate,  as  I  have  found,  that  such  a 
medium  occurs  in  a  solution  of  sulphate  of  copper  and  ammonia. 

954.  Let  a  wooden  box,  a  h  {Jig.  131),  six  inches  long,  two  wide,  and  two  deep,  be 
provided ;  in  the  centre  of  its  top  an  aperture  three  quarters  of  an  inch  in  diameter  is 
to  be  made  ;  the  box  must  be  blackened  interiorly,  and  a  rectangular  prism  of  wood,  c, 
be  placed  in  the  box,  with  its  right  angle  in  such  a  position  that  its  edge  bisects  as  a 
diameter  the  circular  aperture ;  over  this  wooden  prism  a  piece  of  clean  white  paper 
should  be  pasted,  care  being  taken  that,  where  it  bends  over  the  right  angle  of  the 
prism,  it  is  folded  sharp.  So  far  the  reader  will  recognise  in  this  Ritchie's  photome- 
ter, as  described  in  the  Annals  of  Philosophy.  Upon  the  aperture  in  the  top  of  the 
box  a  glass  trough,  g  h,  is  placed  ;  it  is  made  by  drilling  a  circular  hole  an  inch  in  diam- 
( ter  in  a  piece  of  plate  glass  one  third  of  an  inch  thick,  and  then  placing  on  each  side 
(if  it  a  thin  piece  of  plate  glass.  This  forms  a  circular  trough,  in  which  a  strong  solu- 
tion of  sulphate  of  copper  and  ammonia  may  be  enclosed ;  over  the  trough  a  conical 
tube,  d,  six  or  eight  inches  long,  is  placed,  so  that  the  eye  may  see  distinctly,  through 
the  aperture  in  the  top  of  the  box,  the  disk  of  paper,  and  more  especially  its  dividing 
diameter. 

955.  Two  small  lamps,  ef,  are  then  prepared,  of  such  dimensions  that,  when  set  op- 
posite the  open  ends  of  the  box,  their  rays  may  illuminate  the  paper;  they  are  suppo- 
sed to  be  adjusted  so  as  to  shine  with  equal  intensity. 

956.  On  looking  through  the  tube  a  circle  of  blue  light  is  seen,  and,  if  the  lamps  are 
shining  equally,  its  two  halves  are  equally  bright.    At  the  commencement  of  every  ex- 
periment this  preliminary  observation  should  be  made,  and,  if  necessary,  the  proper  ad 
justments  secured. 

957.  Suppose,  now,  it  were  required  to  know  how  much  blue  light  is  transmitted  by 


EXPLOSIONS  BY  A  DISTANT  ELECTRIC  SPARK.  209 

a  given  solution.  A  trough,  g,  is  to  be  provided,  which  may  be  formed  by  drilling  a 
hole  two  and  a  half  inches  in  diameter  through  a  thick  piece  of  plate  glass ;  on  each 
side  of  this  a  thin  piece  of  similar  glass  is  laid,  the  trough  having  been  filled  with  the 
solution  under  investigation.  Troughs  made  in  this  manner  never  leak ;  they  com- 
pletely answer  their  purpose,  and  are  easily  washed  and  refilled. 

958.  Let  the  substance  under  trial  be  a  concentrated  solution  of  bichromate  of  pot- 
ash. Having  adjusted  the  lamps  and  filled  the  trough,  set  it  before  one  of  them,  sup- 
porting it  by  a  proper  foot.  On  looking  through  the  tube  of  tlie  photometer,  if  the  ab- 
sorbent cell  of  the  copper  solution  has  been  previously  removed,  the  circle  of  light  will 
be  seen  very  brightly  illuminated ;  one  of  its  halves  of  a  yellow  tint,  but  not  much  less 
luminous  than  the  other.  The  copper  cell  being  now  restored  to  its  place,  on  looking 
again  through  the  tube  there  is  a  striking  contrast — one  half  of  the  circle  is  of  a  bright 
blue,  but  the  other  seems  totally  black ;  with  solutions  which  cut  off  the  blue  rays  less 
perfectly,  this  blackness  is  of  course  less  intense ;  in  these  cases  the  lamps  are  to  be 
moved  into  such  positions  that  the  two  halves  of  the  circle  are  equally  illuminated,  and 
its  dividing  diameter  invisible.  As  the  eye  is  not  disturbed  with  any  difference  of  col- 
our, the  observation  can  be  made  within  small  limits  of  error.  The  calculation  of  the 
relative  intensities  can  then  be  made  by  the  common  photometrical  law. 

959.  As  this  photometer  is  affected  by  rays  of  the  same  refrangibility  as  those  which 
affect  the  tithonometer,  it  is  clear  that  if  it  be  the  rays  of  light  which  are  operative  in 
the  union  of  chlorine  and  hydrogen,  the  results  given  by  the  two  instruments  should 
correspond  within  certain  small  limits  of  error. 

960.  With  respect  to  the  tithonometer,  I  have  improved  this  instrument  considera- 
bly :  by  shading  it  with  a  glass  case,  which  cuts  off  thermometric  disturbance ;  by  ta- 
king the  observations  through  a  small  telescope,  which  avoids  parallax ;  by  having  the 
scale  movable,  so  as  to  slide  along  the  tube ;  by  making  one  charge  of  gas  last  for  a 
great  number  of  experiments  ;  by  completing  the  tithonization  before  commencing  ;  by 
altering  the  position  of  the  adjusting  wire,  so  as  to  bring  it  nearly  down  to  the  end  of 
the  tube.  The  detail  of  these  changes  would,  however,  detain  me  now  too  long ;  they 
will  be  given  at  some  suitable  time  hereafter. 

961.  I  may,  however,  record  as  a  striking  fact,  that  so  great  is  the  sensitiveness  of 
chlorine  and  hydrogen,  that  a  mixture  will  actually  explode  by  the  rays  of  an  electric 
spark.  In  Chapter  XVI.  I  have  already  stated  that  silent  combination  would  take 
place  under  these  circumstances,  but  more  recently  I  have  had  instruments  repeatedly 
destroyed  by  explosions  resulting  in  that  way. 

962.  It  being  understood  that  the  indications  of  the  photometer  and  the  tithonome- 
ter correspond,  that  they  are  affected  by  rays  of  the  same  refrangibility,  we  are  enabled 
to  proceed  to  the  direct  solution  of  the  question,  and  the  determination  of  M.  Becque- 
rel's  hypothesis. 

963.  A  transparent  medium,  which  absorbs,  to  a  greater  or  lesser  extent,  the  more  re- 
frangible rays,  being  selected,  it  is  required  to  determine  whether,  when  a  given  ray  has 
passed  through  it,  the  chemical  effect  diminishes  as  the  intensity  of  the  more  refrangi- 
ble rays  of  light  diminishes.    According  to  M.  Becquerel,  the  effect  should  be  in  di- 


210 


ABSORPTIVP]  ACTION  OP  MKDIA. 


rect  proportion  to  the  quantity  of  light ;  if,  for  example,  the  ray  lost  one  half  of  its  blue 
light,  the  chemical  effect  should  diminish  one  half,  &c. 

964.  We  require,  therefore,  two  observations :  first,  with  the  photometer,  to  ascer- 
tain how  much  of  the  light  escapes  the  absorptive  action  of  the  medium  under  trial ; 
second,  with  the  tithonometer,  to  ascertain  what  quantity  of  the  tithonic  rays  escapes 
absorption.  If  the  whole  effect  is  due  to  light,  the  two  observations  should  give  the 
same  result. 

965.  Before  giving  the  results  which  I  have  obtained  in  a  tabular  form,  in  order  that 
I  may  be  clearly  understood  I  will  give  a  particular  example.  I  took  some  naphtha, 
the  colour  of  which  was  slightly  yellow,  and  placing  it  in  the  glass  trough  before  de- 
scribed, proceeded  to  determine  its  relation  for  the  more  refrangible  rays  of  light.  This 
was  done  by  adjusting  the  two  lamps  of  the  photometer  till  they  coincided,  then  inter- 
posing the  trough  between  one  of  the  lamps  and  the  end  of  the  photometer.  On  look- 
ing through  the  tube,  a  great  diminution  of  the  intensity  of  the  light  on  the  correspond- 
ing semicircle  of  paper  was  pbserved ;  the  other  lamp  was  now  removed  until  the 
paper  disc  was  uniformly  illuminated ;  the  distance  of  the  two  lamps  from  the  centre 
of  the  box  was  now  measured — they  were  respectively  twelve  and  fifteen  inches ;  but 
the  intensity  of  the  light  is  proportional  to  the  square  of  the  distance. 

(1.)  For  the  light  in  the  unobstructed  beam   225  or  100, 

(2.)  For  the  light  in  the  absorbed  beam  144  or  64. 

Supposing,  now,  that  the  value  of  the  unobstructed  beam  be  represented  by  100,  and 
we  calculate  the  amount  of  light  wdiich  passes  through  the  naphtha,  we  find  it  is  repre- 
sented by  64 ;  consequently,  of  every  hundred  rays  of  blue  light  which  fell  upon  this 
naphtha,  sixty-four  escaped  absorption. 

The  naphtha  trough  was  now  carried  to  the  tithonometer;  first  it  was  determined 
how  many  seconds  it  took  a  given  beam  of  light,  coming  from  an  Argand  lamp,  burn- 


ing steadily,  to  move  the  index  through  one  division. 

(3.)  For  the  tithonic  ray  of  the  unobstructed  beam  31'. 

The  trough  was  then  interposed  in  the  column  of  light,  and  the  number  of  seconds 
required  to  make  the  index  move  through  one  division  determined. 

(4.)  For  the  tithonic  ray  in  the  absorbed  beam  65'. 

But  as  the  lanip  might  have  varied  in  intensity,  or  the  tithonometer  in  sensitiveness, 
the  first  operation  was  repeated  ;  it  gave, 

(5.)  For  the  tithonic  ray  of  the  unobstructed  beam  31'. 


This  process  of  repetition  was  uniformly  resorted  to,  and  the  mean  of  the  two  ta- 
ken.   It  may  be  proper  to  remark,  that  there  was  rarely  any  perceptible  difference. 

966.  It  follows,  that  a  ray  which  could  effect  the  union  of  a  given  quantity  of  chlorine 
and  hydrogen  in  31  seconds,  required,  after  passing  through  the  naphtha,  65  seconds. 

Calculating  on  these  principles  how  many  of  the  tithonic  rays  passed  through  the 
naphtha  as  before,  we  find, 

(6.)  For  the  tithonic  rays  in  the  unobstructed  beam  100. 

(7.)  For  the  tithonic  rays  in  the  absorbed  beam  48. 

Now,  comparing  this  result  (6)  and  (7)  with  the  result  (1)  and  (2)  for  light,  we  find 


ABSORPTION  IN  THE  TWO  CASES  FOLLOWS  DIFFERENT  LAWS.  211 

that  the  absorptive  action  of  naphtha,  of  a  sliglitlj'-jellovv  tint,  is  very  much  greater  for 
the  tithonic  rays  than  for  the  luminous  rays. 

9G7.  Consequently,  it  follows  that  it  is  not  the  light  comprehended  between  the  ex- 
treme blue  and  extreme  violet  rays  v\  hich  brings  about  the  union  of  chlorine  and  hydro- 
gen, but  another  and  invisible  class  of  rays,  which  is  absorbed  by  naphtha  under  a  differ- 
ent law  for  that  for  its  action  on  light. 

9G8.  The  foregoing  example  shows  the  mode  by  which  I  have  obtained  the  results 
of  the  following  table : 

T.\BLE  SHOWING  THE  ABSORPTIVE  ACTION  OF  CERTAIN  MEDIA  FOR  THE 
LUMINOUS  AND  TITHONIC  RAYS. 


Name. 

Number  of  rays  which  escaped  absorption. 

Tithonic  rays. 

Luminous  rays. 

Naphtha   

48 

64 

Sulphocyanate  of  iron  .... 

66 

58 

53 

64 

Red  prussiate  of  potash  .... 

13 

15 

20 

38 

24 

42 

Iodide  of  starch  

40 

33 

Stevens's  blue  ink  

30 

33 

Litmus  water  

16 

23 

From  this  table,  therefore,  we  gather  that  the  chemical  effect  produced  by  a  given  ray 
h;i.s  no  relation  to  the  quantity  of  light  which  is  in  it;  that  a  satisfactory  explanation 
of  the  phenomena  can  only  be  given  by  assuming  the  existence  and  presence  of  an- 
other agent  besides  the  light,  and  to  which  agent  the  chemical  effect  is  due;  that  me- 
dia are  known  which  in  their  absorptive  action  bear  relations  which  are  totally  differ- 
ent for  these  two  agents  ;  and,  finally,  that,  as  prismatic  analysis  has  also  previously 
shown,  no  explanation  can  be  given  of  these  results  by  imputing  them  to  the  agency 
of  light,  we  are  forced  to  admit  the  existence  of  another  imponderable  principle,  the 
same  as  that  which  passes  in  these  papers  under  the  name  of  tithonic  rays. 

969.  In  addition  to  the  results  obtained  from  the  foregoing  quantitative  experiments, 
there  are  other  phenomena  of  a  very  novel  and  interesting  kind,  from  which  we  may 
draw  an  argument  of  overwhelming  force.   The  discussion  of  these  I  shall  now  take  up. 

970.  Early  during  the  last  century,  the  remarkable  appearance  of  phosphorescence 
excited  in  the  Bolognean  stone  and  calcined  oyster-shells,  attracted  the  attention  of  chem- 
ists. DuFAY,  in  France,  wrote  several  papers  upon  it;  and  the  experiments  of  Wilson 
in  England  are,  perhaps,  as  fine  a  specimen  of  philosophical  investigation  as  those  early 
times  can  furnish.  These  results,  which  few  are  now  acquainted  with,  deserve  to  be 
republished. 

971.  To  Becquerel  we  are  indebted  for  one  of  the  most  remarkable  discoveries  in 
connexion  with  this  subject.  He  found  that  the  rays  of  an  electric  spark  which  had  pass- 
ed through  glass  no  longer  preserved  the  quality  of  exciting  phosphorescence,  but 
when  they  passed  through  quartz  they  retained  that  power  unimpaired.  This  result 
is  very  strikingly  shown  by  placing  a  piece  of  colourless  glass  and  a  piece  of  quartz  on 
a  surface  covered  with  sulphuret  of  lime  (oyster-shells  calcined  with  sulphur),  and  dis- 
charging a  Leyden  vial  a  little  distance  off.  The  sulphuret  will  glow  as  brilliantly 
on  the  part  covered  by  the  quartz  as  on  the  uncovered  spaces,  but  under  the  glass  it 
wil.  remain  dark. 


212 


THE  PHOSPHOROGENIC  RAYS. 


972.  Nevertheless,  this  same  sulphuret,  carried  into  the  sunshine,  phosphoresces  power- 
fully under  glass,  apparently  showing  that  there  is  a  difference  between  the  phosphoro- 
genic  emanation  of  the  sun,  for  thus  M.Becquerel  terms  it,  and  that  of  an  electric  spark. 

973.  On  this  radiation,  as  it  comes  from  the  sun,  M.  Bfxquerel  has  treated  in  his  pa- 
per, of  which  a  translation  is  given  in  Taylor's  Scientific  Memoirs  (vol.  iii.,  part  12). 
He  determines  the  place  of  the  phosphorogenic  rays  in  the  spectrum  after  they  have 
passed  through  a  glass  prism,  and  shows  that  the  fixed  lines  which  occur  in  the  chem- 
ical spectrum,  occur  also  among  these  phosphorogenic  rays. 

974.  In  passing,  I  may  mention  that,  at  the  time  I  published  my  account  of  these 
fixed  lines  {Phil.  Mag.,  May,  1843),  I  had  no  idea  that  any  other  chemist  had  seen 
them.  It,  of  course,  soon  appeared  that  M.  Becquerel  had  some  months  previously 
given  an  account  of  them  to  the  French  Academy.  My  result  was  wholly  independ- 
ent, and  without  any  knowledge  of  his.  On  comparing  the  two  papers,  it  will  be  seen 
that  there  is  a  strong  coincidence,  not  only  in  the  manner  of  the  experiment,  but  even 
in  the  very  phrases  of  description.  It  is  this  whic  h  has  drawn  these  passing  remarks 
from  me.  Men  who  are  pursuing  the  same  object,  and  using  the  same  resources,  will 
employ  even  words  that  are  alike,  though  they  speak  languages  that  are  different,  and 
live  thousands  of  miles  apart. 

975.  On  examining  the  plate  given  in  M.  Becquerel's  paper,  I  was  struck  with 
the  close  resemblance  between  the  phosphorogenic  spectrum  and  that  titiionographic 
spectrum  on  iodide  of  silver  of  which  Sir  J.  Herschel  has  given  an  elaborate  account 
(^Fhil.  Mag.,  Feb.,  1843).  As  far  as  the  eye  could  judge,  they  seemed  perfectly  alike  ;  the 
tithonographic  spectrum  in  question  was  obtained  by  me  in  Virginia.  This  coincidence 
was  so  striking  that  it  appeared  almost  certain  that  the  phosphorogenic  emanations  of 
Becquerel  were  the  same  as  my  tithonic  rays :  there  was  the  upper  spectrum  com- 
mencing at  the  line  G,  and  going  beyond  the  farthest  confines  of  the  violet,  exerting  a 
positive  action  ;  there  was  also  the  lower  spectrum,  commencing  at  the  line  F  and  going 
below  the  red  ray,  and  exerting  a  negative  action ;  a  phenomenon  absolutely  the  same 
as  that  traced  on  the  Daguerreotype  plate. 

976.  The  phosphorogenic  rays  that  come  from  the  sun  have  the  same  place  in  the 
spectrum,  or  are  dispersed  by  the  prism  exactly  in  the  same  way  as  the  tithonic  rays. 
To  all  a})pearance  these  may  be  expressions  for  the  same  agent. 

977.  We  must  remember,  however,  that  the  phosphorogenic  rays  of  the  sun  differ 
from  those  of  an  electric  spark.    Glass  to  the  former  is  transparent,  to  the  latter  it  is  not. 

978.  Before,  therefore,  I  can  carry  this  argument  to  the  point  on  which  I  design  it 
to  bear,  it  is  necessary  to  ascertain  the  index  of  refraction  of  the  rays  of  an  electric 
spark  to  which  glass  is  impervious. 

979.  This  I  proceeded  to  determine  in  the  following  way :  At  the  distance  of  six 
inches  from  the  terminations  of  two  blunt  wires,  between  w  hich  the  spark  from  a  Ley- 
den  vial  was  caused  to  pass,  I  placed  a  lens  of  quartz,  the  focus  of  which  for  parallel 
rays  was  six  inches,  and  then  intercepted  the  resulting  beam  by  a  diaiiluagm  with  a 
circular  aperture  in  it  one  third  of  an  inch  in  diameter.  I  had  carsiHl  an  equiangular 
prism  of  quartz  to  be  cut  and  polished  from  a  large  and  perfectly  faultless  rock-crystal ; 


/ 


INDEX  OF  REFRACTION  OF  THE  PHOSPHOROGENIC  RAYS.  0]3 

it  was  cut  transverse  to  the  axis.  This  prism  I  placed  in  such  a  way,  that  in  disper- 
sing the  beam  that  came  through  the  circular  aperture  I  got  rid  of  double  refraction 
and  obtained  only  one  spectrum  ;  this  was  received  on  a  metal  plate,  which,  having  been 
washed  over  with  gum  water,  and  sulphuret  of  lime  dusted  on  it,  offered  a  uniform 
phosphorescent  surface,  which  might  be  set  in  a  vertical  plane.  When  the  spark  pass- 
ed, I  saw  that  tJie  plate  loas  phosphorescing  on  those  portions  where  the  more  refrangible 
rays  had  fallen. 

980.  But  the  transient  light  of  a  Leyden  spark  did  not  last  long  enough,  nor  was  the 
phosphorescence  it  produced  powerful  enough  to  enable  me  to  conduct  the  experiment 
in  a  way  entirely  satisfactory.  I  resorted,  therefore,  to  the  brilliant  light  which  is  ob- 
tained when  a  piece  of  metal,  or,  what  is  far  better,  the  hard  variety  of  carbon  which 
is  obtained  from  the  interior  of  gas  retorts,  is  lowered  upon  mercury  entirely  filling  a 
very  small  open  porcelain  cup,  and  the  continuous  discharge  of  a  voltaic  battery  passed. 
The  battery  used  contained  fifty  pairs  of  Grove's  cells,  but  a  smaller  number  would 
probably  have  been  amply  sufficient.  All  the  remainder  of  the  arrangement  was  as  just 
described. 

981.  As  soon  as  the  light  was  emitted,  I  marked  on  the  sulphuret  of  lime  the  begin- 
ning of  the  red,  the  centre  of  the  yellow,  and  the  termination  of  the  visible  violet  ray. 
Then,  stopping  the  current,  I  examined  on  what  parts  the  plate  was  phosphorescing. 
The  commencement  of  the  glow  was  between  the  indigo  and  the  blue ;  towards  the 
blue  it  extended  far  beyond  the  visible  boundaries  of  the  spectrum  ;  I  could  not  see  any 
divisions  or  points  of  maxima  on  it.  The  surface  of  the  plate  shone  all  over,  except 
in  the  region  of  the  less  refrangible  rays,  and  there  were  the  traces  of  the  negative  ac- 
tion which  M.  Becquerel  has  so  well  illustrated  in  the  case  of  the  solar  emanations; 
rays  which,  however,  were  first  observed  in  the  last  century. 

982.  It  is  necessary  to  remark,  that  the  rays  from  the  voltaic  discharge  resemble 
those  from  an  electric  spark  in  their  inability  to  traverse  glass.  On  this  observation  all 
the  value  of  the  foregoing  experiment  depends. 

983.  But  it  can  nevertheless  be  easily  proved,  that  although  glass  is  impervious  to  the 
phosphorogenic  emanation  coming  from  the  voltaic  deflagration  of  any  metallic  bodies, 
the  observation  applies  to  transient  discharges  only.  A  voltaic  light,  which  lasts  but 
a  moment,  fails  to  cause  phosphorescence  through  glass  in  the  same  way  that  an  elec- 
tric spark  does;  but  if  the  discharge  is  continued,  the  surface  presently  begins  to  glow; 
and  if  maintained  for  several  minutes,  it  shines  as  brightly  as  though  a  piece  of  quartz 
had  been  used. 

984.  The  inability  of  an  electric  spark  to  cause  phosphorescence  is  connected  with 
its  transient  duration.  The  voltaic  light  enables  us  at  pleasure  to  imitate  the  effects 
of  an  electric  spark,  or  those  of  the  sun. 

985.  The  phosphorogenic  rays,  whether  they  originate  in  an  electric  spark  or  from 
the  sun,  occupying  thus  the  same  place  in  the  spectrum,  and  even  exhibiting  the  same 
peculiarities  as  the  tithonic  rays  on  the  iodide  of  silver,  we  have  next  to  determine 
whether  this  is  an  apparent  or  a  positive  identity. 

986.  Professor  Henry,  of  Princeton,  rea  l  a  paper  before  the  American  Philosoph- 


214 


PHYSICAL  INDEPENDENCE  OF  THE  PHOSPHOROGENIC  RAYS. 


ical  Society  in  May,  1843,  in  which  he  discussed  all  the  leading  mechanical  properties 
of  the  phospliorogenic  rays,  and  among  other  important  experiments,  made  some  witli 
a  view  of  determining  this  particular  question.  A  Daguerreotype  plate  and  some  sul- 
phuret  of  lime  were  simultaneously  exposed  to  the  sky  ;  the  plate  was  stained,  but  no 
effect  was  produced  on  the  lime.  A  Daguerreotype  plate  and  some  suiphuret  of  lime 
were  exposed  to  the  light  of  an  electric  spark ;  the  lime  was  observed  to  glow,  but  no 
impression  was  produced  on  the  plate.  When  the  plate  was  exposed  to  a  succession 
of  sparks  for  ten  minutes,  with  a  sheet  of  mica  interposed,  an  impression  was  made. 
Lime  exposed  to  the  moon  did  not  phosphoresce,  but  a  sensitive  plate  under  the  cir- 
cumstances is  said  to  be  stained.  In  view  of  these  different  facts.  Professor  Henry  ob- 
serves, "  These  experiments,  although  not  sufficiently  extensive,  appear  to  indicate  that 
the  phosphorogenic  emanation  is  distinct  from  the  chemical,  and  that  it  exists  in  a 
nuich  greater  quantity  in  the  electric  spark  than  either  the  luminous  or  chemical  radi- 
ation." 

987.  From  Wilson's  experiments,  it  appears  that  he  was  aware  that  when  the  phos- 
phorescent surface  is  warmed,  so  as  to  hasten  the  disengagement  of  light,  the  moon- 
beams may  be  found  to  have  left  traces  of  action  upon  it ;  feeble,  it  is  true,  but  never- 
theless very  apparent.  We  have  seen,  also,  that  the  peculiarity  of  an  electric  spark  is 
due  to  its  transient  duration.  Before,  therefore,  a  final  decision  can  be  obtained  on 
this  point,  we  are  required  to  examine  the  effect  of  the  tithonic  rays  and  phosphoro- 
genic emanation,  under  circumstances  which  are  precisely  similar  as  to  intensity  and 
time. 

988.  For  the  transient  rays  of  an  electric  spark,  quartz  is  transparent,  and  glass  is 
nearly  opaque.  Having  prepared  a  bromo-iodized  silver  plate  so  as  to  be  exceedingly 
sensitive,  I  set  in  front  of  it,  at  the  distance  of  about  one  third  of  an  inch,  a  disc  of 
quartz  and  one  of  crown  glass,  of  equal  thickness  ;  and  between  a  pair  of  copper  wires, 
the  interval  of  which  was  three  eighths  of  an  inch,  I  passed  the  spark  of  a  Leyden  vial 
fifteen  times  ;  the  distance  between  this  spark  and  the  sensitive  plate  was  about  two 
inches.  On  mercurializing,  the  plate  was  deeply  whitened  all  over,  equally  so  through 
the  glass,  through  the  quartz,  and  on  the  uncovered  spaces ;  but  a  spot  of  sealing-wax 
which  I  had  put  on  the  glass,  left  its  shadow  on  the  plate  beautifully  depicted,  as 
were  also  the  edges  of  the  glass  and  the  quartz.  The  two  discs  overlapped  one  an- 
other to  a  certain  extent,  but  the  corresponding  portion  of  the  silver  plate  was  as  deeply 
stained  there  as  anywhere  else. 

989.  Next  I  put  a  surface  of  suiphuret  of  lime  in  the  place  of  the  Daguerreotype 
plate,  everything  remaining  as  before.  On  passing  fifteen  sparks,  the  lime  phospho- 
resced powerfully  under  the  quartz,  but  not  under  the  glass,  so  that  the  difference  be- 
tween its  shadow  and  that  of  the  spot  of  wax  could  not  be  distinctly  seen. 

990.  For  these  reasons,  therefore,  I  adopt  the  view  expressed  by  Professor  Henry, 
that  ihe  phosphorogenic  emanation  and  the  tithonic  rays  are  distinct.  Under  the  same 
circumstances,  glass  to  the  one  is  transparent,  to  the  other  it  is  opaque. 

991.  Now  upon  what  sort  of  evidence  is  it  that  M.  Melloni  is  universally  admitted 
to  have  established  the  physical  independence  of  light  and  heat  1    Was  it  not  by  show- 


thp:re  are  more  than  three  imponderables.  215; 

ing  that  rock-salt  is  perfectly  transparent  to  calorific  rays,  that  glass  is  much  less  so, 
that  Rochelle  salt,  alum,  and  sulphate  of  copper  are  almost  opaque  '!  It  is  surely  impos- 
sible to  confound  the  phosphorogenic  emanation  of  an  electric  spark  with  its  rays  of 
light;  the  latter  pass  perfectly  through  glass,  the  former  do  not.  So  far  as  the  eye  can 
distinguish,  an  electric  spark,  the  rays  of  which  have  passed  through  glass,  differs  in  no 
respect  from  one  the  rays  of  which  are  received  directly  into  the  eye.  If  we  consider 
the  constitution  of  such  a  ray,  previous  to  and  after  its  passage  through  glass,  the  eye 
can  discover  no  difference;  but,  as  respects  the  phosphorogenic  emanation,  there  was 
something  existing  in  that  ray  at  the  first  of  these  epochs  which  had  ceased  to  exist  in 
it  at  the  second  ;  a  something  not  having  the  quality  of  communicating  any  impression 
to  the  organ  of  vision  ;  and  that  which  we  cannot  see,  surely  no  man  will  acknowledge 
to  be  light. 

992.  But  the  reader  may  inquire.  What  has  all  this  discussion  of  the  characters  of  the 
phosphorogenic  emanation  to  do  with  the  existence  of  the  tithonic  rays  as  a  fourth  im- 
ponderable ?  A  few  words  will  show.  From  these  considerations  and  experiments, 
we  have  arrived  at  the  conclusion  that  there  exist  in  the  beams  of  an  electric  spark 
invisible  rays,  which  are  therefore  totally  distinct  from  light.  They  occupy  the  same 
spectrum  region  as  the  tithonic  rays  which  decompose  iodide  of  silver  ;  their  leading 
character  is,  that  glass,  which  is  transparent  to  the  rays  of  light,  is  opaque  to  them. 

993.  But  the  admission  of  this  fact  breaks  down  at  once  the  doctrine  of  a  trinity  of 
imponderables,  and  compels  us  to  enlarge  our  list  of  those  living  forces  of  chemistry. 
The  great  obstacle  which  is  in  the  way  of  admitting  the  tithonic  rays  as  a  fourth  im- 
ponderable, is  in  the  circumstance  that  it  would  impress  a  very  serious  change  on  that 
science,  and  apparently  afford  an  argument  of  weight  against  the  mathematical  theory 
of  light.  I  believe  that  some  great  generalization  will  hereafter  prove  that  all  these 
imponderables  are  modifications  of  one  primordial  principle.  I  also  believe  that  some 
capital  experiment  will  hereafter  show  that  the  forty  different  metals  we  are  acquainted 
with  are  merely  modifications  of  one  or  two  more  simple  forms ;  but  these  are  things 
that  we  are  unable  to  deal  with  now ;  and  viewing  the  experiments  which  have  been 
made  in  the  last  few  years,  not  as  mathematicians,  but  as  chemists,  all  men  must  ac- 
knowledge that  our  prevailing  doctrines  of  the  nature  and  number  of  the  imponderables 
are  liable,  before  long,  to  undergo  a  very  serious  modification. 

994.  The  admission  that  the  phosphorogenic  emanation  and  light  are  principles 
differing  from  the  tithonic  rays  and  from  each  other,  relieves  us  of  much  difficulty  in 
increasing  our  list  of  imponderables.  If  these  principles  differ  thus  intrinsically  from 
one  another,  the  question  comes  home  to  us,  What  are  they?  If  electricity,  and  heat, 
and  light  are  three  recognised  imponderables,  are  not  the  tithonic  rays  a  fourth,  and  the 
phosphorogenic  emanation  a  fifth  ? 

995.  In  view  of  this,  I  would  suggest  the  propriety  of  ceasing  to  call  these  last  by 
the  epithet  of  emanations,  and  of  giving  them  the  more  appropriate  name  of  phospho- 
rogenic RAYS. 

996.  And  now,  what  appears  to  become  of  M.  Becquerel's  hypothesis,  that  all  the 
different  effects  we  have  been  considering  are  due  to  hght,  and  are  presented  to  us 


21.6 


CONCLUSION. 


under  different  aspects,  because  everything  depends  on  the  nature  of  the  receiving  sur- 
face ;  that  it  is  the  same  principle  which  affects  the  eye  as  light,  decomposes  chloride 
of  silver  as  a  tithonic  ray,  and  makes  sulphuret  of  lime  shine  as  a  phosphorogenic  ray ; 
that  the  difference  is  not  in  the  radiant  principle,  but  in  the  surface  on  which  it  is  re- 
ceived ?  To  go  no  farther  in  a  discussion  which  has  already  extended  this  chapter  too 
much,  if  the  agent  is  the  same  in  all  cases,  and  the  difference  perceived  is  due  to  the 
receiving  surface,  how  is  it  that  a  ray  of  light  which  has  passed  through  a  piece  of 
transparent  glass  can  no  longer  excite  phosphorescence  in  the  sulphuret  of  lime  1  Can 
we  escape  the  conclusion  that  the  ray  has  had  something  removed  from  it,  or  has  had 
some  modification  impressed  on  it,  or,  in  short,  that  something  invisible  to  the  eye  has 
been  taken  away  1 

997.  To  my  mind  these  considerations  are  conclusive,  and  I  therefore  regard  the 
tithonic  rays  as  constituting  a  fourth  imponderable,  and  the  phosphorogenic  rays  as  a 
fifth. 


THE  END. 


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I'hih  III 


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